Rated 4 out of 5 stars
To get this to work on FF4, download the .xpi installer file, open with an unzip program, open the install.rdf file in a text editor, change the maxVersion to 4.0.*
Then open the closebutton.xul file and change the toolbarbutton id to "close-button" and the class to "tab-close-button"
Thanks for testing it in FF4 and giving me a push to finally update it.