How to disable mouse back and forward shortcuts?

Solution 1:

Type about:config into Firefox's address bar
If you haven't previously disabled it, a warning page about the dangers of changing settings that you don't understand will pop up. Read it, then click "I accept the risk!"
In the about:config search bar, search for mousebutton. You should find two preferences, mousebutton.4th.enabled and mousebutton.5th.enabled, both set to true. Double click each of these to set them to false.
Restart Firefox probably. IDK that's what I did, don't know if it's necessary.