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.
Williamo's Blog 
Thanks! This fixed it
Glad to hear it! =)
Thank you so much for this, fixed my problem straight away! I just set the dpi to 96 like my system is set to and worked a treat.
Thanks!
Finally… you saved my day.
this worked great! thanks again!!!
Thanks everyone! =)
This solution did not solve my problem!
Er, well, it might but for some reason I am unable to change the value. The number is automatically set at “-1” and no matter what integer I change it to, positive or negative, it reverts straight-away back to -1.
Please help. The fonts only appear large on some sites, usually the header.
I also have a problem that, on log-in, the characters are so huge they’re bigger than the login and password box.
Actually this did fix it. Thanks a lot!
I didn’t realize all other tabs had to be closed.
Also, on mine it looks better set to 120; I’m using a 24″ HDTV as my monitor.
I still have the problem of super-huge fonts at the login screen. Any suggestions?
I’m running Ubuntu 8.04.
TONS of thanks for this tip
Thank you all again for your comments on this post!
@HojPoj – I had that problem too with the huge fonts on the login screen, but only at pretty high resolutions (I think Ubuntu is assuming that it’s compensating for super-tiny fonts on a tiny high-res screen). Which resolution are you seeing these huge fonts at?
@HojPoj: Oh, I just thought of something; did you check the Gnome Appearance Properties, under the Font section? There should be an option to change your X server DPI there…
Thank you, thank you, thank you
Been looking for that setting for so long
Thanks! It worked for me….its Mandriva linux…