@GVP

Thanks for the hint but the site says "This is effectively an unsigned version and thus cannot be installed for Firefox" and Firefox really refuses to install it, any other idea ?

I do not use Thunderbird and would only need it for Firefox but see above

Well, I just uninstalled the addon and now Firefox seems to use the general Windows font settings I set longer time ago, rather too big in some places (address bar etc.) but necessary in some other programs very probably and better than too small anyway so maybe this will be a solution for some other people, too