Userscripts
TamperMonkey is a pretty handy browser extension that makes it easy to write a bit of code to add or alter the functionality of a web page. It's basically like a simple, fast way to make a browser extension. These "mini-extensions" are known as "userscripts". Here are some of the userscripts I use, including the ones I've written myself.
DuckDuckGo: Google Jump by modula t.
Quickly jump from DuckDuckGo search results to a Google search for the same query with Alt+g
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EmuParadise download script by Eptun
Restores the rom download links on emuparadise.me.
Wikipedia: Use Desktop Site by modula t.
Automatically redirect to the desktop version of Wikipedia (in case you get a link to the mobile version).