I'm using Firefox 30.0 on all of my Mac computers. I've used a 3rd party backup & file synchronization program for many years to successfully sync the Firefox (FF) Profiles folder between 6 different computers, all running Mac OS 10.6.8. I've recently upgraded one of them to Mac OS 10.9.4 and found that the Profiles folder was now in a different location. But no problem, I only had to tell the backup program where to find the Profiles folder under OS 10.9.4 and things seemed to work well for a couple of weeks, synching between FF Profiles on computers that are running different versions of the Mac OS. Then yesterday something happened with FF on the OS 10.9.4 and I had to go through troubleshooting to retire the old Profile and create a new one for me to continue to work with FF. When I finished working I synched, as usual, to an external HD. That external HD also can run Mac OS 10.6.8 and I actually run my iMac from that external HD. The problem came when I booted the iMac from that external HD and tried to start FF. I was told that FF couldn't find my Profiles. As a result I couldn't start FF on the iMac under OS 10.6.8. The Profiles folder on the iMac does contain a folder named hf38mmri.default-string_of_numbers (I've made-up this name), which apparently was the new Profiles folder created as a result of restoring the OS 10.9.4 FF to its default settings. FF on the iMac couldn't open without the old Profile, named qr64trz.default (I've made-up this name too), so I copied the qr64trz.default folder over from another OS 10.6.8 computer and put it into the Profiles folder on the iMac. I could then start FF on the iMac and post this question via FF on the iMac. So my plan right now is to leave both the qr64trz.default and the hf38mmri.default-string_of_numbers folders within the FF Profiles folder and to use the 3rd party synch program to still synch or backup (whichever action is appropriate) between the 2 operating systems. It seems that FF under OS 10.6.8 needs that qr64trz.default folder while the OS 10.9.4 FF needs the hf38mmri.default-string_of_numbers folder for its Profile. Does that make sense as a plan or should I be doing something else?