What would you like to see from Firefox to make it more headless and scriptable? Are there specific usecases you're interested in supporting?

I'd love to be able to modify JS at runtime on random websites. Too often there's a bug, or a "feature" that prevents me from using a service, that I could fix by removing an event or something in the JS code.

That's what development tools are for. Or Greasemonkey/Violentmonkey.

As far as I know neither Firefox nor Chrome allow you to modify the JS prior to execution without a plugin. You can run random JS, sure, but you can’t monkeypatch.