I also hacked the install.rdf and installed the IE Tab addon to Firefox v3.6... didn't work. The install went OK, all the options are there (tab behavior, the icons, menu item, etc) but when it tries to render in IE you just get a black window. Bummer.
@feetome: try renaming the .XPI to .ZIP then open. Just rename it back when done. If that won't open it, you might have a corrupted download.