Thank you! This works just as advertised. TabWheelScroll was a must-have for me. I would've foregone using Firefox completely without this feature.

Didn't seem to work when first installed, but like another reviewer said it was because I tried it while hovering over the tab bar first instead of the page. The second because I was on the add-ons manager page, so I opened a few new tabs to check it out. Works great!