You can change the firefox menu fonts as follows:
1. Create an empty text file and name it userChrome.css;
2. Cut and paste the following text into userChrome.css:
font-size: 13pt !important;
3. Copy userChrome.css to /home/<username>/.mozilla/firefox/<8 character random string>.default/chrome
The next time you open firefox, the menu/gui fonts will be 13pt. If that's too big, edit userChrome.css to 12pt, 10pt, etc. If it's too small, edit to 14pt, 16pt, etc. You have to close and open firefox before your edits will take effect.