TLDR; Bookmarklets are an easy way to have custom javascript to support your testing that sync across browsers.

It took me quite a while to start using Bookmarklets but now that I’ve started… ooh, just try and stop me.

A Bookmarklet is basically some JavaScript code that you store as a bookmark in your browser so you can click on it and instead of being taken to a new link something else happens.

http://blog.eviltester.com/2017/04/a-quick-intro-to-bookmarklets.html