Ello gents, I have been trying to track down a solution to this one for quite some time now. This issue has occurred on two different Macs running OS X 10.10.4 Yosemite (2013 MBP and a 2011 Mac Mini). Firefox 39.0 is installed on both of these systems, and for some reason .mp3 files will not play within Firefox from gmail. They must be downloaded, and then played using an audio player on the computer. Mp3 files work on other pages, just not from gmail when trying to listen to them. I have attempted starting Firefox in safe mode, verifying that permissions for gmail allow audio playback and popups, reinstalling and trying on different systems, but thus far I cannot this to function properly. I have this solution for Windows based systems: https://support.mozilla.org/en-US/questions/982412, but am unable to locate anything for the poor old Macs. Any direction or solutions are greatly appreciated.