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What Operating System do you use?
OS X can be 10.5.8, 10.6.8, 10.7.5, 10.8.5 or 10.9.2
I have the same problem. OS X 10.9.2 and Firefox 28. When I tried to go back to 12, I realized it is no longer available, I can only get 13, which leaves my pages blank.