The latest version appears to be 44.6 -- have you tried that? https://addons.mozilla.org/en-GB/firefox/addon/theme-font-size-changer/

" Not available for your platform"

Fedora release 16 (Verne)

Thunderbird 17.0.2

There seems to be a problem with v. 44.2 on Linux, so I suggest you try an earlier version.

I went to the "earlier version" link but I couldn't figure out how to download the xpi file.

I was able to hit the install butt on load it into Firefox, but still don't know how to load in Thunderbird.

Right click and select "save link as" would be the logical choice. Unfortunately Mozilla act as if Firefox is the only product using their web sites. It is after all their only product, the others being community projects. Having dropped Thunderbird and Firefox OS all they have left in their stable is Firefox.