I'm actually thinking your System Accessibility settings have the Font Size set a bit large ... but in case it IS Thunderbird here are a Few Things to check: Under Thunderbird Menu choose Preferences In the Menu on the left choose Display Click on the Formatting Tab at the top Check that the Default Font size is 12 or less ... Then Click the Advanced ... Button just after the size selector and Check the Checkbox for Allow messages to use other fonts. Click OK then close the preferences tab. See if that helps.

Okay, moved all the preference and related profile directories in ~/Library/, reinstalled thunderbird, rebooted, renamed the profile and preference directories back, started thunderbird and it snapped back to normal.

I am not sure if it as a thunderbird or os bug.

thanks, j.