baltpeter

URL bar doesn't work for me either but saving it as a bookmark does. Here's a quick recording: https://cdn.baltpeter.io/tmp/firefox-bookmarklet.gif

Huh, that's curious. I'm running 69.0.2 on Ubuntu and it works just fine here.

browser.urlbar.filter.javascript is set to its default of true, I didn't touch that (or any other related settings as far as I remember).

Sorry, in that case I'm out of ideas as well.

Edit: Also works in a completely clean and new profile for me without any settings or addons whatsoever. Strange.

This works fine for me for setting the video speed in Firefox:

javascript:(function(){(document.getElementsByTagName("video")[0]).playbackRate=prompt("Enter%20desired%20speed%20(e.g.%201.5)","1.5")})()

I also remember having some issues with porting bookmarklets from Chrome to Firefox, I believe the solution was wrapping them in an IIFE like here.