I don't think it's supported in the latest version of Firefox on Android. It supports only 11 extensions total and Greasemonkey is not among them:
https://addons.mozilla.org/en-US/android/search/?promoted=recommended&sort=random&type=extensionAt any rate, Mozilla seems to be very intent to kill Firefox if you look at the latest Play Store reviews so I won't be putting too much effort into it. Once the dust settles and there is a clear path to make the extension work in the long run - maybe.