Comment 2 for bug 584632

I had this same problem and it drove me nuts. My problem is that when I reply to an existing email, the font will be "Helvetica, Arial". This is what's listed when I go to Edit --> Preferences --> Composition --> General --> HTML --> Font. But if I click away from the body of the email, even if I click to another part of the composition window such as the subject box, as soon as I click back to the body, the font will change to some kind of serif font. It says "Variable Width" in the font box. I think it's whatever font is listed in Edit --> Preferences --> Display --> Formatting --> Fonts --> Default font. My configuration says the default font is "serif."

If I type a completely new message, I don't seem to have the problem as much.

My solution was to simply set the font settings in the locations I mentioned above to the same font. So I went to Edit --> Preferences --> Display --> Formatting --> Fonts --> Default font, set it to Arial, and it hasn't happened again, yet--it wasn't that long ago that I made the change, so we'll see how well it works.