If, like me, you want to see plain text emails in a monospace font like God intended, you probably went to this screen:
And then hit “Advanced” button in the Fonts section:
And chose the fonts you enjoy. After a while, though, you’ll realize that some messages don’t look right. They don’t look right at all. Messages that should look like this:
Instead, look like this:
Instead of your beloved Source Code Pro, Thunderbird is inexplicably using the depraved and wholly offensive “fixed” font. This is apostasy. Fortunately, it’s easily remedied. Go back to the “Advanced” section under Edit > Preferences > Display, and change the “Western” selection at the top to “Unicode”, and make your font selections again:
This is because when you thought you were setting the font for all messages, you were only setting the font for emails with ISO-8859-1 encoding. Emails, unfortunately, come in another flavor: UTF-8, or Unicode. You have to set the font for both, because Thunderbird likes wasting your time.