Surprised nobody already suggested some kind of mouse gestures extension. I use foxy gestures and once you learn a few gestures (starting with closing a tab and undoing it) it becomes hard to live without it! Gesturefy is also a popular alternative.
Firefox
A community for discussion about Mozilla Firefox.
Genuine question: How are gestures faster / more efficient than clicking a button, or using keyboard hotkeys? Or is this mainly for notebooks, tablets and smartphones?
Compared to clicking a button I would say a lot faster as you don't have to move your mouse to start initiating your gestures. It's maybe more marginal compared to a short key. Still your hand is already on the mouse ready to execute a gesture. Personally I find it really convenient and for me it's the second most useful add-on after ublock. When using a browser on other computers that don't have these I find it really tedious and I am shocked at how much ad there is.
Behind the Overlay.
https://addons.mozilla.org/en-US/firefox/addon/swift-selection-search/
"Swiftly access your search engines in a popup panel when you select text in a webpage."
I don't want to say its name, cause I worry (maybe unnecessarily) about making it a target, but there's a downloader for YouTube that's the only extension I've ever paid for.