There’s also TamperMonkey (closed source) and ViolentMonkey (also MIT license like GreaseMonkey). I prefer ViolentMonkey. I believe there was a reason many years ago where either Tamper or Grease monkey didn’t work for like a week which is why I switched, but I believe they’re all equally good.
Manifest v3 might make it harder to get going on chromium browsers in a months time, but looks like you’re on Firefox - shouldn’t be problem there.