Mail & Newsgroups

then Message Display

Try the "Bigger" setting. I tried it on mine, with Mozilla 1.2.1, but it just blows up the browser on Windows, gah! So I don't know if it works.
Also try changing to the variable width font there.

But I do believe that it uses the system settings for the font in the Mail forms, if you change that one to 24 points, it should use it. If not, holla at the Mozilla development team to make a tweak to change it. Maybe 1.3 will have it?