Eric Meyer Go to the source
If you upgraded Thunderbird and discovered that the fonts used to display messages suddenly changed, and worse still, you were unable to get all messages to obey your font display settings, then this post is most likely for you. Here’s what happened to me: I upgraded to Thunderbird 3.1, and suddenly all my messages were in a font I didn’t recognize or appreciate. I insist on seeing only the plain text version (technically, the text/plain part) of all my e-mail; and what’s more, that it be displayed in a monospace font. Courier 13, in my case. So I made sure “View > Message Body As” was still set to “Plain Text”, which it was… Read the rest here