I encounter this problem this days, only in FireFox, the other software works fine. The cause was a buggy driver for my mouse (which is actually an touch pad). My driver was ALPS ("Alps pointing device"). Driver version was 7.0.xxx. I found a new version which ALSO DID NOT SOLVED the problem - 7.7.1602.501. It is the newest I have found. I downloaded it from here.
In spite of the fact that driver is rated to be used in Windows 7 32 bit, it works fine on my Win XP.
Anyone to know a solution for me?