I'm having two problems with FF4beta4. "Phantom" tabs. When I close a tab, open a few other tabs, and then close them, the original tab pops back up.
One time, I closed that tab, and then clicking the "new tab" button, changed the URL bar back to that web site, and the page was still there. Clicking around some more, my only tab visible on the tab bar (AT forums) disappeared mysteriously, and all I had left was a non-functioning new tab button in the tab bar, and the phantom website was still there.
Granted, I was still downloading a file off of that web page, but still, why won't FF let go of that page once I close the tab? And losing my existing tab was a major glitch.
Another problem that I've had is that Hotmail doesn't work with FF 4. I can login to MS Live/Hotmail, but when I click on inbox, I cannot click on any messages to look at them. Seems like a JavaScript glitch.
Whether this is because Hotmail doesn't recognize FF4 yet, or whether FF4's new JS engine is b0rked, I don't know.
Edit: Actually, my problem was with FF4beta3. I updated to FF4beta4 a little while ago, and I just tried Hotmail, and it worked this time.
Edit: Wierd. On the Win7 taskbar, with the mouse-over window preview, the two missing tabs that I lost were still there, just not on my tab bar. Clicking those windows brought them back onto my tab bar. I've also had other times, when the Win7 taskbar totally lost the mouse-over preview of my tab windows, although they were still on my tab bar, I couldn't mouse-over and open them.