I can't find anyone with this problem of not being able to install greasemonkey since installing Firefox version 21 on my android. My ultimate goal is to combine my fitness pal logging with weight watchers so that mfp will calculate ww points. It can be done but only through using greasemonkey. Any help is greatly appreciated :)
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.