Hello jgieschen,

Holding down the Ctrl key and rotating the mouse wheel might be why this is happening.

Would you try this please :

Type in the address bar about:config and press Enter (promise to be careful, if asked)

Type in the search bar mousewheel.with

Look for the preference mousewheel.withcontrol.action

and set its value to 1

Then close and restart Firefox.

Any luck  ?

If not : change the value back to its default (which is 3).

Did you try to reboot the computer?

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10