I, and many others, have had the same problem. I found the same question which was posted a year ago on another site. There were 74 people who clicked "I have this problem, too," but there were no replies for fixes. However, I just downloaded firefox 4 and the problem is solved. Since you already have the newest version, I suggest you uninstall then reinstall firefox 4.

Uninstalling and reinstalling Firefox 4 didn't solve the problem.

Although this method worked for me in the past with the same problem, it didn't work this time. Maybe it'll work for you. Click help>troubleshooting information>open containing folder. Within the folder are 2 or 3 files named places.sqlite. Rename each file places.sqlite old, places.sqlite-shm old, places.sqlite-wal old. The next time you open firefox it will generate new files in place of them. Like I said, it didn't work for me this latest time, but I've had the same problem in the past and it worked then.

I've had the same problem after upgrade from FF3. Solution:

[https://support.mozilla.com/de/questi.../803069#answer-167598]

Wow, so the "solution" is to erase my whole browsing history? Well, luckily, IE9 seems to be quite good, so I don't really need to deal with this kind of stuff.

Thank you for your assistance.