I have that problem. Is this new? As in the past 24 hours or so? I just finished a clean install of Windows 7 and found this suddenly happening.

Update: It's fixed for me. If you have by any chance an add-on called Pentadactyl installed you should update it (to its latest nightly). I do believe it was the cause. Anyway, for me it fixed itself automatically. Some add-on received an update, i guess.