I’ve had the Font Changer installed in Thunderbird for quite a while. On start-up today, my fonts were back to the original tiny size, and there are no Font Changer menu items now. I first tried to disable/re-enable the add-on. During disable, I got three errors… two about buttons being empty (probably the menu links that are missing), and another long one that I didn’t fully understand.
I then removed it completely and re-installed from the current version. It still doesn’t work. Between the time it worked (yesterday) and didn’t work (today), I didn’t update anything on my Macbook. So unless something installed automagically without me knowing it, nothing has changed.
Had precisely the same problem. Finally uninstalled and reinstalled Theme Font Size Changer and solved that problem, BUT created a new problem: some of my emails are displaying in giant-size type and I don’t see a way to fix this.