It seems that FireFox 3.6 defines navigator.mimeTypes. Sadly, this is what gears_init.js is using to determine if the browser is Safari. (see http://code.google.com/apis/gears/gears_init.js - Line 57)
It looks like the browser detection needs to be updated to ensure that it doesn't get a false positive in the Safari detection code, as it causes an error.
