The open source nature of Firefox helps to have these type of possibilities: optimize the browser engine to your hardware.
On MOOX you can download optimized version of Firefox and Thunderbird, precompiled for different CPU for maximum stability and speed (from AMD processors family to Pentium, Pentium 2, Pentium 3, Pentium 4, Pentium M and Xeon family).
Interesting to test... 
Just some hours ago, Mozilla has released the 1.0 RC1 version of Firefox, the Release Candidate that will be like a preview to the big launch of the official 1.0 version in November (9th November the announced date).
You can have a ChangeLog here... if you are planning to test the RC1, it's important that you submit all your feedbacks and bugs to the Bugzilla Forum, so they can be correct or revisited for the official 1.0.
What is good is that seems that all your extensions used on the previous 1.0PR wil continue to work, as reported on forums. Just open /%appdata%/Mozilla/Firefox/Profiles/randomname/extensions/Extensions.rdf and change all instances of maxVersion="0.10" to maxVersion="1.0"... all will be ok!