Firefox 3 still too slow on your low end machine?
I know how you feel. After all this hype about the most downloaded software 'browser' in the world, I suddenly felt that I needed a fresh and alternative perspective.
Swiftweasel is the most suitable and capable candidate I've come across so far.
Swiftfox project is an optimized build of Mozilla Firefox. Swiftfox has builds for both AMD and Intel processors. The 184.108.40.206 release is based on Firefox 220.127.116.11.
The trunk is the most cutting edge of all the builds and is where Firefox 3 is being developed.
Branch builds are built from a code base that has been tested much more than the trunk and therefore should be considered to be more stable.
Existing Firefox plugins should work with no additional configuration if you use the installer or deb file.
How To Install
You can find installation instructions for Ubuntu and Debian over at Debian Admin.