Easily fixed to work with Firefox 3.6 Rated 5 out of 5 stars
This is easily fixed to work with Firefox 3.6 . . . in the extension there are 4 files (about.xul, AdvancedDialog.xul, c4c.xul, & options.xul) simply remove the copyright symbol from line 5. That's it and it works again.
I have sent feedback to the developer with this info, so maybe it will be updated shortly since it is such a simple fix.