I've just discovered
Firefox 3.1 BETA 2 portable (
release notes) is out, so I just had to go and test (Follow the instructions
here to run the beta concurrently to your 3.0 installation).
As most people are likely aware, Firefox 3.1 will include a new
private browsing mode (known to many as "porno mode") which will see your browser forget everything that you did while it was enabled. This feature is now available in BETA 2. From the tools menu, select "Private Browsing". Firefox will revert back to your current session when you're finished.
Many will be thanking the gods that 3.1 now includes preliminary support for
Web Workers. That is, multithreading support in the javascript engine. This will see the departure of days where you browser would lock up if it encountered a particularly nasty script somewhere that required more computation than your machine could provide in a reasonable amount of time.
TraceMonkey, Firefox's new and improved javascript engine, is enabled by default so you should see some added zippyness™ when visiting Gmail et al.
They've removed the new tab switching behavior shown
here in post 1 due to negative user feedback.
The support for HTML 5 elements is now completely stable (as far as my experience goes). Give them a try yourself
here.
Ehtyar.