Firefox version 21 installed easily but greasemonkey won't install - error saying not compatible with ver 21
The error message in this case is misleading: it's not that Greasemonkey is incompatible with that version number, rather that it's incompatible with Firefox for Android. The extension mechanism in Firefox for Android is somewhat different to Firefox on desktops, so add-ons for desktop Firefox won't work (in general) on Android.

To find out if/when Android support is planned, or to help out, you could start by contacting the gresemonkey-users mailing list.