An extremely irritating bug in Thunderbird. Don't even remember how long it has been doing this; when I am typing a message, if I so much as switch away from the Thunderbird window (to look something up for example), when I come back to the window it will switch the font size to 1-2 points smaller than what it had been doing before. I have not selected any formatting settings, and had been typing in it's default type size. Merely switching out of the window should not perform a type size change.

I have ever had to just highlight the entire message (if TB has completely fubarred the font), delete it from TB, paste it into a plain-text editor to remove all formatting, and then paste it back in.