I have had exactly the same problem (in all beta version also of Firefox 4) and only today discovered that it was the safe browsing toolbar from Bullguard.
So safe it stops all web searches from working meaning no chance of getting malware!
I have disabled it and all is working well.
As ever a security suite has a bad glitch that they don't seem to want to address.
Now my browser in unsafe thanks to their malfunctioning toolbar