Do you think that Firefox is too slow for your internet experience. Maybe you need to have a special firefox built especially optimised for your architecture. You may try out SwiftFox which has various builds firefox which is suitable for your particular processor. There are Ubuntu deb files too available.
Swiftfox has builds for both AMD and Intel processors. The 1.5.0.6 release is based on Firefox 1.5.0.6.
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. The 1.8 branch is where Mozilla develops Firefox 2 which will be the next major Firefox release.
The 1.8.0 branch is the most stable build, second only to the release builds. This is where Mozilla tests security patches and other minor fixes before they release the next Firefox update. If you don’t want to wait for the next Firefox update to get those security patches then this branch is for you.