Firefox 3.5

[Lemur]

It's noticeably faster on my Wintel box. Note that Firefox now starts up with tabs visible at all times, which I find irritating. (Why are you chewing up screen real estate when it isn't necessary, you beast?)

To turn it off in Windows: Tools→Options→Tabs→Always show the tab bar.

-edit-

Ars Technica also has a big write-up of all the goodies included in the upgrade. Worth a read.