After struggling with this lovely error ("XML Parsing Error: not well-formed" while pointing to a nonexistant location in my code) for a while hacking around on Wereboy, I became a little frustrated. So I created a clean profile and added the extension, where it worked fine. Turns out that profiledir/XUL.mfasl will apparently keep around uninstalled plugin chrome then later confuse itself, resulting in this sort of breakage. You have to delete/move it then relaunch Firefox to force a rebuild.