Works in FF 3.0.11 Rated 5 out of 5 stars

You have to open the xpi with e.g. 7zip, WinRar, Winzip etc. and edit the install.rdf.
Write:
3.*
where you find something like 2.*
Save the changes and zip it again to the xpi.
Now you can install it.
You also can use Mr. Tech Toolkit to disable the compatibility-check, but this is unclean.