mp4 files include videos encoded with H.264 codec. Since H.264 codec is not a free codec, it is not included in iceweasel. Iceweasel needs a third party application to play H.264 encoded files. After some research, I came up with a solution to play my mp4 videos under debian jessie, because converting the mp4 files to ogg format is not option for me. This link tells the reasons and supported formats in a detailed way. Of course I advice you to use open standards in web however in my case, I had to use mp4 files.
media.fragmented-mp4.exposed -> true media.fragmented-mp4.enabled -> true media.fragmented-mp4.ffmpeg.enabled -> true media.fragmented-mp4.gmp.enabled -> true media.mediasource.mp4.enabled -> true
pref("media.fragmented-mp4.exposed", true); pref("media.fragmented-mp4.enabled", true); pref("media.fragmented-mp4.ffmpeg.enabled", true); pref("media.fragmented-mp4.gmp.enabled", true); pref("media.mediasource.mp4.enabled", true);
Last job is to install the necessary decoder which is installed with ffmpeg2theora package.
apt-get install ffmpeg2theora
That is all. It works.