ProCon Latte + Firefox 6 (Final) = WORKS! Rated 5 out of 5 stars
I opened the xpi with 7-Zip and changed the install.rdf like Pro24 stated above to maxversion 7.0 and it now works! Thanks for the tip Pro24! I changed it to 7.0 in case I want to try the 7 alpha/beta. I tested it with version 7 alpha 2 and it works great.
If anyone needs a working version for Firefox 6 final you can get it at the link below. Unzip it then go into your add-ons and install it manually.