You learn something new every day. Today I learned about insect reproductive systems, and also that document.designMode is a thing. Both seemed pretty interesting, but only the latter let's you make any text on any web site "editable".
Open up the dev console and type document.designMode="on", and marvel as you can type over any text on a page (you even get spellcheck in Firefox). Setting document.designMode="off" restores the page to a read-only mode more strongly sclerotized than a head or thorax. It's like contentEditable, but for everything.
I don't know how this feature managed to escape me for so many years: it was introduced a billion years ago in Internet Explorer, has been part of Firefox since (at least?) version 3, has full support pretty-much everywhere, and seems designed for singular purpose of pranking your office coworkers.
Just my luck to discover it right after offices stopped being a thing.