Firefox recently changed to a 64bit only browser type, similar to what Google Chrome had done a few months ago. At this time, we do have a work around for this issue:
•In the address bar type: "about:config"
•A message will pop up for dangers of the these options agree to them.
•Search for "xpinstall.signatures.required". It should have a value of True.
•Right click on "xpinstall.signatures.required"
•"Toggle" the value will change to false.
Mozilla is currently working on a permanent fix, but this should work for you in the mean time.
I hope this helps!!
Sling Media Technical Support