I had a very specific problem with fonts in Mozilla Thunderbird on my new MacBook Pro, turns out a font was incorrectly identifying itsself as Arial, one of the basic system fonts.
To identify the problem font, use Spotlight to search for the font name that isn’t working, in my case Arial. You should see the fonts appear, you can then open them with FontBook and remove them.
Thunderbird is back to normal for me.
Hope this helps someone else.