I was having similar problem to fix it run MozBackup and go through the wizard then uninstall and reinstall Firefox then run MozBackup again and restore everything. If that does not work try creating a new profile Instructions Here and manually copying every file from your old profile except prefs.js (you can find your profile folder by going to about:support and clicking"Open Containing Folder" under Profile directory before creating the new profile) then restart Firefox and you shouldn't have any problems with it anymore.

MozBackup is Windows only--doesn't work on my Mac.

If you just copy your profile folder it does the same thing or uninstall Firefox and in the uninstallation wizard tell it to "Leave data" that should also work.

If you click on the "down" arrow in the Google search box and then move your cursor to the desired term in search history while still left-clicking on your mouse, and then hold the cursor over the desired search term for a couple of seconds, it works.

Clearly a bug that I hope they will fix.

i replaced firefox4 with previous version. that solved problem and is good to be back on the old user friendly. thanks for replies

thanks, i did the same thing - went back to old firefox!