Special PSA for the zero people aside from me who have suddenly noticed this extremely specific technical problem: if you're using X-Mouse Button Control and the latest Firefox Nightly/Windows 10/something update seems to have broken the ability to use the mouse wheel to scroll in Firefox, go to your XMBC settings and enable "Disable Scroll Window Under Cursor" in the Options tab of the Default profile (and then remember to click "Apply", the thing I always forget when testing settings).

For everybody who has not had this exact problem just now, I actually highly recommend XMBC for messing with how the mouse operates and adding hotkeys to buttons 4 and 5. I promise it doesn't break like this too often.