There is something new and totally unexpected in that 3.1 I just installed on my Mac Book Pro through the update mechanism: the monospace font I'm using, I've been using for the last twenty (even 24) years reading plain/text email, Courier, is gone and Thunderbird seems to force me to switch to Menlo... I really wonder who had that excellent idea. Suddenly, my eyes are not used to the email font any more, and reading a message is less automatic.

Changing the default font is not a harmless thing. It impacts readability, accessibility, habits. It should never be done w/o asking the user. I consider this change as a strategic mistake because TB asked me nothing.

Menlo 12Courier 13
Menlo12Courier13

I'm immediately switching back to Courier through the Preferences panel. And I would recommend switching back TB3.1 distros to the original font.

Update: the first comment below makes me wonder if people will really understand why this is a bad change. My font choices are mine, strictly mine. The software does not have the right to force a change here w/o prompting me. Would Firefox force a change on someone's userContent.css or in the preferred background/foreground colors? Clearly no. So why is it acceptable for Thunderbird?