Possible Fix for Everyone's Problems... Rated 4 out of 5 stars
I've discovered that Firefox must be set up to "Remember History" under Preferences -> Security select Remember History (Mac). If you're browsing privately or have things setup to delete cookies every time you close Firefox this plug-in won't work. I like to browse without history so I got everything working by allowing cookies to be installed until I got the plug-in working then I customized Firefox such that it kept the cookies from Garmin and deleted all others.