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