BIG THANKS TO USER "e12884"

IT REALLY WORKS. THANK! THANKS!

Worst day of my life... this mourning Thunderbird on my both MACs has been automatically updated and now this Addon in not working. Tried to roll back to the old version but that does not helped. My eyes are bleeding when I reading my Emails folders (((
Please help me, love Thunderird, but my eyes are crying ;((

@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

I've seen your Theme and fonts resize has been updated today (6th January 2018) but still not working :-(
Please fix it asap.
Many thanks

I found a workaround for the loss of TFSC that works on Firefox.

1. Create a "userChrome.css" text file: https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/

2. Add the code found here for "Global UI font": http://kb.mozillazine.org/Pane_and_menu_fonts
Change parameters as desired.

3. For bold font add this line before the end symbol: font-weight: bold ! important;
Change parameters as desired.

4. Save file and restart Firefox. There may be other parameters available but these were all I needed,

I too was having the same problem as many others have posted about (extension stops working and no icon in toolbar even when installing old verisons). I was able to fix it by installing an even older one, Version 46.5. There still is no icon to click, but you can access it from the Tools menu in Firefox. It seems to work fine now, just make sure to disable automatic updates!

I've used this add for years and in general it is a godsend.
Periodically however it just falls over.
Last night my fonts reverted to their microscopic version.
I checked and found a new release.
As in the past I remove all old versions and then install per the instructions.

For TB the TF&S change should be visible in 2 places.
I have zero.
I am showing V62 as enabled and installed but that's all.
As said a truly great add on that really should be a part of TB BUT ONLY WHEN IT WORKS.

Was a great program but FF is determined to make the system font so small you can't read it, so this no longer works, due to the extensions support going away. FF is becoming all the things I hated about chrome. Hope at some point in the future this program will be useful again.