There is a hidden feature in one of Firefox flags to switch between flags by having the mouse hover over the tabs bar, and use the mouse wheel.
For this, you go to "about:config", and enable "toolkit.tabbox.switchByScrolling", and restart Firefox.
Please make it more official, via the settings, and also make it wrapping, so that I could go from first to last tab, and the opposite.
I had this feature on various web browsers (Maxthon, Cent browser), and it would be great to have it officially on Firefox.
Attached screenshot of how it looks like in the settings of Cent browser