This is the unofficial optimized firefox page
http://pryan.org/mozilla/firefox/
If you follow the discussions, you'll find mmoy's unofficial patches-more coming all the time.

which version is best to run on a Celeron 500 and a Pentium 4 2,4Ghz?

hi Sebastian,
FF i do not know for Your pc; (i do not have a celeron and a p4 )
but on the site mentioned they make descriptions for what cpu & which purposes and they are. If You follow links from that page You find more details sometimes.

k-m, at the moment ulfs test k-m, & eyes-only test k-m, and Dorian`s 2 test k-m.
at my pentium 500 dorian's static is fastest;
and static is fastest on the high end machine of Jan too.
& fitted with a variant of Your menus they are best ; o )
gruss guneter

p. s. wenn du die FF links abklapesrt findest du zuweilen infos für was die FF sind.
& es wird bald ein schnellerer inoffizieler k-m mit deinen menus sein.