Originally Posted by GGJstudios
Which browser and version are you using?
Have you tried resetting it, clearing your cache and cookies?
What browser add-ons, plug-ins or extensions are you using?
Can you be more specific about what mouse settings have changed?
What kind of mouse do you have?
Are you using any mouse configuration software?
How to maximise your MacRumors troubleshooting experience
I use Firefox and Chrome, both the latest versions for Mac (I'm not near my MP so I can't give exact versions). Only extensions I'm using are the StumbleUpon bar and PDF Viewer for Firefox. Haven't checked Safari because I don't use it and don't have any bookmarks or cookies in that browser.
I'm using a Magic Mouse. Mouse settings went completely back to default, i.e. Tracking speed, scrolling direction, multi-touch gestures, etc... This all occurred on a fresh boot, I typically shut my MP down at night and during the days that I'm not working at home.