Firefox

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.

Behind the Overlay.

I have compiled the best recommendations in a list: https://lemmy.world/post/14173277

I still don't care about cookies

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."

uBlacklist is great for hiding horrid search results.

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.

Tab Center Reborn (and some css to hide the top tab bar) for sidebar tabs.