There are two things you can try.
From the main menu in Kde, go to System - Gnome, and start the Gnome control center. Click on the Fonts icon, an set the font sizes there (only 4 items to set).
You can also create a file called userChrome.css in ~/.mozilla/firefox/<profile>/chrome. That file can also control the appeareance of fonts in the viewing window of Firefox. You may have a userChrome-example.css that you can copy and edit to your liking.
For more information, see http://www.mozilla.org/unix/customizing.html