Custom Site JS allows you to run your own (per url or domain) custom JavaScript on sites.

You can add up to 5 megabytes of your own JavaScript that will automatically be run on a page for you every time you visit a site.

All you need to do is enter the snippet of code you want to run in the box. You can add as many as you like.

Each snippet of code runs as soon as the page is loaded. This means you don't have to click a bookmark or bookmarklet each time you visit a site.