Rated 2 out of 5 stars

Shown as not compatible with current version of TB (version 91.1), but Font size change seems to work OK without the extension by making a change in TB Preferences as suggested by Ruben.

There's no need to install add on to make "config editor" appear as tab, just go to preferences and config editor is at the bottom. Open this and search for Font, then refine search as font.size.systemFontScale (by default is 100). Edit and changed the number as required.

I used 120 and Thunderbird is now usable for me!