Updated after user answer:

Thanks a lot for the detailed explanation of the issue. I promise that I will go through each one and investigate it thoroughly to find the bug.

Regarding the other 2 issues:
1. The scrollbar is awfully small on purpose, let's just say by design (I thought that a non intrusive scroll bar is better because I expected most people to just use the mouse scroll )... of course, it doesn't have to be like that, it can be stupid small and I totally understand that. I will make sure to add an option to configure it to be just normal. Regarding the interference from the popup with the scrollbar... this one also on purpose because it looked a little out of place to move it near the scrollbar. Not really sure who to approach some things having in mind that there's no real solution to place everything on screen and not affect something else. I will think about it. The fact that the mouse can be glitchy sometimes... I'm afraid that can't be something from be but most probably from the addon popup window (if in the Tab View it doesn't happen then that's for sure)

2. Need to investigate that further, because it seems really weird

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Are you sure they are not just in reverse order ? Depending on certain Firefox setting each user has, tabs can be restored in reverse order or in the normal order (If for example you activated new tabs to be opened next to the current one or similar settings). This is not something I can control myself since each user can affect the order of the tabs. I just simply tell firefox to open all the links no matter how many they are. If there are 1000 tabs I tell Firefox to open all of them in the right order they were saved. I wish there was something wrong with the way the Addon was built but trust me there's really no issue. There can be multiple settings (from other addons or just Firefox settings) that can mess up what I tell the browser to do.
That's why I added the "Reverse restore order" feature in the Options. Please check that one to see if order will be preserved.
The addon knows to open even more than 10000 tabs, but depending on the number you want to open I'm not really sure if Firefox can really handle it (more exactly if the PC hardware can handle it the correct way). The thing is, if there's really a bug in the way Firefox opens 100 tabs when I tell him to do that I can't really do anything about that.

Anyway, I will investigate this again, but there's simply nothing wrong with the addon, I do what I must (I mean.. really... I go through every single tab and request Firefox to open it... it's not like I can't really do it properly), but if tabs are not opened then I probably have to check if there's any limitation regarding what the addon API can do, or if Firefox might miss some messages from the addon and I will try to find some workarounds.

Regarding your request to show the number of tabs next to each session. Yes, sorry about that, I will try to land it in the next update (the feature is planned but I kind forgot about it).

Regarding the rating, please have in consideration that there are extremely few people that really have hundred of tabs opened and restored so even you if are totally right the review might not be in line with most of the users. I really try to improve it constantly and even if I do something wrong I try to fix it right away. Having support from the community it's what drives me to continue to improve it :). Issues are fixable, but sadly bad ratings remain no matter what I do :(. No, I'm not asking to change your review, I just wanted to point out that it may not be a very accurate review for whoever intends to use the addon in the future.

Update
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I updated the addon. The new version should not have the same problem. I'll try to add more advanced feature in next releases to make sure that storage can't be deleted by such an error.
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I'm really really sorry about the issue. I think I found a problem related to why history session might disappear suddenly and I will try to have a fix today, or later tomorrow. I really appreciate that you didn't go straight to 1 star and I promise that I will do my best to fix it as soon as possible. Will update this comment once the fix is ready.
Sadly I discovered after I launched the addon that addon storage seems not to be that reliable, having in mind that even by reinstalling the addon you can lose everything. I think I will add a form of secondary backup (or even some form of manual backup) to make sure things are more reliable.
I know many people rely on the auto-save function but the addon is mainly developed for manual saving of sessions as bookmark. I kinda made the wrong decision to provide an auto-saving feature at some point and sadly that created a lot more problem than I expected.
Again, really sorry, I will do my best to make sure things like this can't happen again.

Yeah, sadly once the addon API changed Mozilla removed the possibility to store additional data alongside a bookmark and all the functionality that was based on that (pinned information, and other tab session information) was no longer going to work. I do have a solution in mind to provide this back, but sadly it's not something that can be as reliable as bookmarks and I'm not really sure if on the long run is not worse to implement something that may work only until you delete the addon and then just lose that data suddenly because it was saved in addon storage.

Regarding the "click cancel". Well that's not really what it happens there but I get why you thought that. That dialogue it's actually meant for you to edit the name of the session (which is already saved) so basically clicking cancel will just leave the session with the default name. The same edit box will show if you just go over any other session and edit it's name. I totally missed that it an create confusion if I show the edit box right after saving a session, that's a great observation and I'll think about it if there's anything that ca be done to improve the experience so that saving steps may be more clear. So this is not really a issue... everything works as it should... but I will think about the whole saving process to see if there's anything I can improve to be sure things can get confusing.

I'm not really sure I do understand the 3'rd statement :
"And it would be great if there was an option to manually save session in bookmarks or, like autosave session, in sqlite database in addon data storage folder."
So, saving in bookmarks is manually... or you wanted to say automatically save ?
Regarding addon storage... well that's seems to be problematic since it's not really reliable. Addon storage is there only if you have the addon installed. If by mistake (your or because of Firefox) the addon is removed, add the data associated with that specific addon is deleted right away. For example this is what happenes with the auto-saved sessions, they are simply deleted by Firefox when the addon is removed. So storing anything valuable in addon storage it's not something I can do right now , probably after I add some form of full backup, but even then it seems to me that the addon storage is really good for nothing because it give users the apparent idea that data is save but it's just isn't.

If you have any other feedback you can contact me directly over email (use the Feedback button from the addon menu) or if you have any feature request (or issue) you can also send it to https://github.com/ReDEnergy/SessionSync.

Thanks for detailed feedback (and for using the addon)

Thank you as well for using the addon, and for the great rating :)

Thanks for using the addon. I'm really glad that you find it useful.

Thanks for the update...
Now, there's no perfect software out there and I'm pretty sure there can be a few bugs into my addon as well but I try to fix them as soon as possible if they really brake the functionality. If you ever have problems again please don't have any reservation in using the feedback option from the addon to contact me because It's way faster to discuss about certain issues and understand better how to deal with that and I can assure you that I do everything possible to fix problems very fast.

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Hi. Any chance You can contact me directly over email to solve this issue. It may happen that some bookmarks were saved wrong somehow (it's fixable). If the page is really one of those links then that's not an issue, but If the page is suppose to be a web page then maybe it was saved wrong. Anyway I need to know what URL did the page had, if it's not a restricted URL then probably like I said maybe after a Firefox update or add-on update something was saved wrong. There was another user that had a similar problem and we fixed it.

Thanks a lot! For me, it's amazing when others find it as useful as myself because I never quite expected that something that I build for myself will be as useful for others as well :D I'm really glad you like it.

Thanks for the review. Sadly the addon is not working right now on mobile (it has some issues with the API from mobile and I couldn't get it to work a while back - I should try again maybe I can make it work now). There should be no way to install it on Firefox mobile and I really have no idea how you managed to install it that because it's not supported and there's no official version for mobile so this is quite strange that you managed to do that.
Of course everything you saved into bookmarks you can access them on the mobile device (as long as you have configured Firefox Sync account to sync bookmarks).. It's not perfect but at least if you really need access to the saved session you can open them.

Thanks a lot for the kind words and the nice review. It matters a lot to me to know that there people out there that really appreciate it and it gives me another good reason to continue to improve it further.

Hi and thanks for the review.

The thing is I didn't add a delete option the the "Active snapshot" because basically once the auto-save timer is triggered everything will be rewritten so deleting something from there can only work for as long as the auto-save timer is not triggered.
Now... I seriously should add the option to delete some other stuff... like closed windows from there because they can keep stacking up and there's no way to remove them right now.
Also I can definitely add the option you're talking for all the other saved sessions from the History Tab since there those will not be changing at all and you should be able to do whatever you want. I will let it available for the "Active snapshot" as well.. just in case someone wants to save things right after deleting a saved tab but as I was saying if the re-saves is triggered during that time... everything will be there again and I can't really do something about it.

Drag-n-drop was not implemented fully for History sessions but I will try to add support for as many things as possible even there.

Will try to implement this as soon as possible. If you remember about it when using the addon, I hope you will remember about this review as well and update it if it really becomes "a five-star extension" :)

Thanks for using the addon.

** Update **
Thanks for all the details. I will try to add the save and close feature in the next update. There will be 2-3 configuration settings to check for it initially to make it as you wish since I still need to consider everyone's ideas when implementing this but as long as it's a one time thing should be fine.
Regarding the sidebar... that requires a lot UI restructuring and that might take some time but I would consider this as well for future updates. Thanks.

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Can you please provide more information about how would you expect the addon to work when all tabs are closed ? All tabs from the current session or all windows and just leave one window (in case the session saves all windows and not just the current one). There's a thread here (https://github.com/ReDEnergy/SessionSync/issues/33) related to this issue, maybe you can provide your idea as well so that I can have multiple input on how to solve this.
I put tooltips for the toolbar menu but I suppose that you still need to move over them to understand them exactly what they do. Sadly, Firefox limits to 800 px the width of the addon so I can only put there labels for a few actions before I'm left with no space and that's why I used only icons. But, It's planed to have an option to also show the text along the icon, or only the text. I will try to provide this option as soon as possible.

So, If i fix those 2 issues then would you consider it a 5 star addon? Isn't there anything else? Because it's kinda hard for me to get how these only 2 issues make the addon a 3/5 star. I know it's hard to please everyone, but just think that it usually takes a lot of time to develop such an addon and the only reason why a developer would continue it is because there are users that support it, not financially, but by understanding that even if it's not perfect it can be improved. Of course, don't get me wrong, I'm not asking you to change the rating, but only to consider that low ratings can really hurt addons on the long run and are not really helping their development. Yes a review is also personal, it doesn't have to reflect everyone preferences, but it's really hard for me as a developer to add support for everything from the start. In time I might be able to do so but community support is valuable. Got a lot of request and ideas from users (https://github.com/ReDEnergy/SessionSync/issues) but implementing all of them is really challenging because many can be contradictory or were never in the original scope of the product.

Hi. Thanks a lot for review. I really have no idea why I didn't think about that. I will add an option to do that in the next release. Probably using a context menu I will be adding more common features for interacting with open tabs.