Could that be related to moving the mouse? Like a "Mouse Gesture"? You didn't bother providing Troubleshooting Information as requested, so we can't view what addons you have installed in Firefox.

Do you have that problem when running in the Firefox SafeMode? http://support.mozilla.com/en-US/kb/Safe+Mode Don't select anything right now, just use "Continue in SafeMode."

If not, see this: http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+theme...

Intsead of him (or them) i can post what i m using right now.

As Extensions i m using: Feedback 1.0.3 Java Quick Starter 1.0

As Plugins: Adobe Acrobat 9.1.0.163 Java Deployment Toolkit 6.0.220.4 Java(TM) Platform SE 6 U22 6.0.220.4 Microsoft DRM 9.0.0.4503 (Drm Store Netscape Plugin) Microsoft DRM 9.0.0.4503 (DRM Netscape Network Object) Shockwave Flash 10.1.102.64 Windows Media Player Plug-in Dynamic Link Library 3.0.2.629

As Theme: Only Default one.

After a bit long search and tryouts ive found the issue.

• Disabling Horizontal Scrolling*

If you re using a laptop with touchpad there are 2 regions at the bottom of your touchpad and right part. Used to scroll pages. Horizontal scrolling can cause this problem with some external usb mouse so disable it. It can be done from your mouse options under Controll Panel. (virtual scrolling etc. for my laptop LG e500)

Hope this helps. Worked for me.