I am using this addon in thunderbird too and there is a fix available and it works but only for thunderbird where I applied it without problem. Please see:
https://addons.mozilla.org/en-US/firefox/addon/theme-font-size-changer-for-tb/reviews/
It is not possible to install adjusted addon back to the firefox maybe due to signing addons which is required by firefox.

But there is another important aspect: the creator of this addon implemented time check into source code which was needed to remove
to ensure proper function in Thunderbird. Please see details in forum link I posted above.
Many thanks mrx2020, HackerKitty, P. Maigron for spotting this. And of course the same "time check" is in source code for firefox too. I do not judge anybody just inform abou the issue which prevents me from using firefox and thunderbird properly.

Edit: posible workaround for firefox thanks Eckard :
http://forums.mozillazine.org/viewtopic.php?f=38&t=3035148
my userChrome.css:
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
@namespace html url("http://www.w3.org/1999/xhtml");
@namespace svg url("http://www.w3.org/2000/svg");
* {
font-size: 14px!important;
font-family:Comic Sans MS !important;
}