Update: Here’s a list of more tips for Firefox 3 on Linux, including how to make your bookmarks toolbar text smaller, and remove the down arrows from bookmarks folders! Super Awesome Firefox 3 Tips! (Linux)
For those of you seeing strange and unreasonably large font sizes in Firefox 3 on Ubuntu 8.04, try setting the “layout.css.dpi” (via about:config) to 72 or 96. Either of those values should set all fonts to a reasonable size.