If you are using it and notice problems, it's best to log an issue or let us know at greasemonkey-dev (and be clear that it's with this version).
This release contains just one important fix:
- Fix memory leak related to GM_registerMenuCommander. (#1578)
I bungled a bit of the release, as early commenters below note. I also hit a bug in the Mozilla Add-ons site in the process, so this release is now called 0.9.22, and officially there is no version 0.9.21. Sorry!