A Chrome and Firefox extension that mounts your browser tabs as a filesystem on your computer.

This gives you a ton of power, because now you can apply all the existing tools on your computer that already know how to deal with files – terminal commands, scripting languages, point-and-click explorers, etc – and use them to control and communicate with your browser.

Now you don’t need to code up a browser extension from scratch every time you want to do anything. You can write a script that talks to your browser in, like, a melange of Python and bash, and you can save it as a single ordinary file that you can run whenever, and it’s no different from scripting any other part of your computer.