Rated 1 out of 5 stars

I attempted mrx2020's fix. I have the latest Thunderbird. When I went to the Add-Ons Manager and checked the page for Theme Font & Size Changer, it said "THUNDERBIRD USERS PLEASE SWITCH TO THE NEW THUNDERBIRD VERSION BY BELOW URL:
https://addons.mozilla.org/en-US/firefox/addon/theme-font-size-changer-for-tb/"
I went there, but it said "Not compatible with Firefox Quantum. This add-on is not available on your platform" and would not let me right click on "add to Firefox" to download the .xpi.
I smelled a typo, so I edited the URL to be https://addons.mozilla.org/en-US/thunderbird/addon/theme-font-size-changer-for-tb/ -- Extracting this, I did see bootstrap.js.
I edited bootstrap.js like the instructions said, NOTE! You have to comment out the lines with //, NOT ## like in HTML! then PeaZipped it (an xpi is just a zip file, you change the extension from .zip to .xpi) I literally did name it whatever.xpi. Then did an Install From File in Thunderbird and it appeared to install successfully. Yay! I then restarted Thunderbird. Be sure to delete the old, non-working Theme Font & Size Changer in Add-Ons after doing this.

If you don't use // for commenting out the lines, it will act like it installs just fine, but you won't be able to see it. Be sure to use // !!

NOTE: We users should not have had to do any of this.

This review is for a previous version of the add-on (62.0).