Started development on a Firefox addon again:

Adding an onclick HTML tag/property (however you call that) - doesn't fire at all.

Adding an event listener - says no elements with the class I'm using (checked, it's correct) exist, doesn't fire.

Well, fucking great 😐

Make sure the events are added when the dom is built. Either in an document loaded event as @jespersh mentioned, or in a script tag at the end of the body. I think the easiest way to achieve first method is by adding a function call to the onlaod attribute of the body tag.

make sure your script is not running in the wrong iframe

Be sure not to add () or your function call turns from a function call into its return value

I did a workaround and that works now (better option in the end anyways).

Now stuck on cancelling requests, no matter what I do (tested enough to know that it DOES reach that code), it doesn't cancel the requests I want it to cancel 😣