Probably because it's used for scrolling the tab strip when the amount of tabs is so high that their width stops shrinking and further overflowing tabs are clip

Probably because it's used for scrolling the tab strip when the amount of tabs is so high that their width stops shrinking and further overflowing tabs are clipped from the strip area. Relatedly, if you use Tree Style Tab there's an extending extension that adds proper mouse wheel scrolling: https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab-mouse-wheel/ https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...