I just tested 2 MP4 files exported from Lightroom 5.7 from Nikon D7000 and D750 cameras and both played in Firefox 33.1 on Win7 using a direct file link to location on my C drive. Also tested a few sample MP4's off the internet and they all worked.
I suspect that your PC/Mac does not have the correct plugin installed to play the files.
Though this question should be directed more at a Firefox forum if that is your issue.
Otherwise, please give us more details on how you are trying to view these files in Firefox after they leave Lightroom. Do you have a web gallery or something you have created?
What version of Lightroom/OS do you have?
And what are the video export settings?
Bruce, Thanks for replying. I use Lightroom 5.6. My problem is that the video, which can be seen at http://oldeworldelace.com/Custmers-lace-panels.html cannot be played in its entirety (only through the first 30 or so second before it starts over) on anyone's Firefox browser. Please try it. It works fine on Chrome, IE and even on my C drive. An Adobe tech rep says it works on his Safari browser. It just cannot be played on Firefox. The program obviously is of no use to me unless it can be played by everyone on all browsers. I would appreciate it if anyone can tell me of a workaround because the slideshow is great. Otherwise perhaps someone can tell me of another software program that can produce the same quality but in all browsers. Thanks for listening.
I am really not sure now where your problem lies but does not seem to be a incompatibility with MP4's.
I was unable to play the slideshow in its entirety with either IE11, Chrome or Firefox. Each of them would play it to a different point at which time it would either start over or show a black screen. This all happened if I pressed play immediately upon loaded your webpage.
However, if I let the video load the buffer for a while and then start playing it then I was able to view the slideshow almost to the end even in Firefox.
So my suspicion is that it is having a video buffering/bandwidth problem from the source server vs incompatible MP4 playback issue with any of the browsers.
You could try saving the slideshow out in a smaller resolution/size choice to see if that helps.