Over at Betanews they have tested the latest alpha release version of Firefox 3.6 and came to the conclusion that the new version increases performance by 22% compared to the latest official release version Firefox 3.5.3. This performance does not come near the performance of Webkit based browsers(google chrome) yet but it shows dedication to close the gap between the browsers.
It remains to be seen on the other hand if most users will notice the 22% performance increase in Firefox 3.6 Beta or if it is just more a figure without real values to everyday users. Performance is definitely not everything. The fastest web browser in the test, Google Chrome 3, does not pass the Acid 3 test which Opera 10 for example does.
I use google chrome 3.xxx on windows 7 RC, and it does have some issues with page rendering.