We had some students who took a video with their phone and wanted to post it on an html page. The html code we used was:

<video controls="controls" width="400">
	<source src="Robot.mp4" type="video/mp4">
	Your browser does not support the video tag.
</video>

In Chrome, this worked fine. But in Firefox, it would show like this.

When that happened, I went to Tools -> Web Developer -> Browser Console. I got a message like this:

HTTP "Content-Type" of "video/3gpp" is not supported.  Load of media resource http://www.example.com/Robot.3gp failed.

This message was strange because I wasn’t posting a file called Robot.3gp, but one called Robot.mp4. So on my server, I checked the file.

$ file Robot.mp4
Robot.mp4: ISO Media, MPEG v4 system, 3GPP

So whatever created this file, actually made a 3gpp file and not a legit mp4. However, I found that you can convert a 3gpp file to mp4 using ffmpeg. I renamed my Robot.mp4 file to Robot.3gp and then converted it.

$ ffmpeg -i Robot.3gp -ab 64k -ar 44100 -strict -2 Robot.mp4
ffmpeg version 2.8.12 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 4.8.5 (GCC) 20150623 (Red Hat 4.8.5-11)
  configuration: --prefix=/usr --bindir=/usr/bin --datadir=/usr/share/ffmpeg --incdir=/usr/include/ffmpeg --libdir=/usr/lib64 --mandir=/usr/share/man --arch=x86_64 --optflags='-O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic' --extra-ldflags='-Wl,-z,relro ' --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libvo-amrwbenc --enable-version3 --enable-bzlib --disable-crystalhd --enable-gnutls --enable-ladspa --enable-libass --enable-libcdio --enable-libdc1394 --disable-indev=jack --enable-libfreetype --enable-libgsm --enable-libmp3lame --enable-openal --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libvorbis --enable-libv4l2 --enable-libx264 --enable-libx265 --enable-libxvid --enable-x11grab --enable-avfilter --enable-avresample --enable-postproc --enable-pthreads --disable-static --enable-shared --enable-gpl --disable-debug --disable-stripping --shlibdir=/usr/lib64 --enable-runtime-cpudetect
  libavutil      54. 31.100 / 54. 31.100
  libavcodec     56. 60.100 / 56. 60.100
  libavformat    56. 40.101 / 56. 40.101
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 40.101 /  5. 40.101
  libavresample   2.  1.  0 /  2.  1.  0
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  2.101 /  1.  2.101
  libpostproc    53.  3.100 / 53.  3.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'Robot.3gp':
  Metadata:
    major_brand     : 3gp5
    minor_version   : 0
    compatible_brands: 3gp5isom
    creation_time   : 2017-07-26 17:45:04
  Duration: 00:00:20.23, start: 0.000000, bitrate: 5135 kb/s
    Stream #0:0(und): Video: h264 (Baseline) (avc1 / 0x31637661), yuv420p(tv, bt470bg/bt470bg/smpte170m), 720x480, 5005 kb/s, 30 fps, 30 tbr, 90k tbn, 180k tbc (default)
    Metadata:
      rotate          : 90
      creation_time   : 2017-07-26 17:45:04
    Side data:
      displaymatrix: rotation of -90.00 degrees
    Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 123 kb/s (default)
    Metadata:
      creation_time   : 2017-07-26 17:45:04
[libx264 @ 0x1e6d1e0] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 AVX2 LZCNT BMI2
[libx264 @ 0x1e6d1e0] profile High, level 3.0
[libx264 @ 0x1e6d1e0] 264 - core 148 r2699 a5e06b9 - H.264/MPEG-4 AVC codec - Copyleft 2003-2016 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'Robot.mp4':
  Metadata:
    major_brand     : 3gp5
    minor_version   : 0
    compatible_brands: 3gp5isom
    encoder         : Lavf56.40.101
    Stream #0:0(und): Video: h264 (libx264) ([33][0][0][0] / 0x0021), yuv420p, 480x720, q=-1--1, 30 fps, 15360 tbn, 30 tbc (default)
    Metadata:
      creation_time   : 2017-07-26 17:45:04
      encoder         : Lavc56.60.100 libx264
    Stream #0:1(und): Audio: aac ([64][0][0][0] / 0x0040), 44100 Hz, stereo, fltp, 64 kb/s (default)
    Metadata:
      creation_time   : 2017-07-26 17:45:04
      encoder         : Lavc56.60.100 aac
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
  Stream #0:1 -> #0:1 (aac (native) -> aac (native))
Press [q] to stop, [?] for help
frame=  607 fps=178 q=-1.0 Lsize=    1593kB time=00:00:20.22 bitrate= 645.1kbits/s    
video:1408kB audio:162kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 1.439324%
[libx264 @ 0x1e6d1e0] frame I:4     Avg QP:19.34  size:  9992
[libx264 @ 0x1e6d1e0] frame P:171   Avg QP:22.56  size:  4317
[libx264 @ 0x1e6d1e0] frame B:432   Avg QP:24.06  size:  1534
[libx264 @ 0x1e6d1e0] consecutive B-frames:  3.3%  2.6%  8.4% 85.7%
[libx264 @ 0x1e6d1e0] mb I  I16..4: 28.5% 58.4% 13.1%
[libx264 @ 0x1e6d1e0] mb P  I16..4:  4.2%  9.2%  1.8%  P16..4: 37.9%  7.7%  5.0%  0.0%  0.0%    skip:34.2%
[libx264 @ 0x1e6d1e0] mb B  I16..4:  0.3%  0.5%  0.1%  B16..8: 42.5%  2.7%  0.4%  direct: 1.4%  skip:52.2%  L0:48.5% L1:48.6% BI: 2.9%
[libx264 @ 0x1e6d1e0] 8x8 transform intra:59.6% inter:80.2%
[libx264 @ 0x1e6d1e0] coded y,uvDC,uvAC intra: 32.2% 41.0% 4.4% inter: 7.5% 10.5% 0.3%
[libx264 @ 0x1e6d1e0] i16 v,h,dc,p: 33% 24% 26% 18%
[libx264 @ 0x1e6d1e0] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 10% 16% 45%  3%  9%  3%  9%  2%  3%
[libx264 @ 0x1e6d1e0] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 13% 14% 27%  3% 18%  4% 16%  2%  3%
[libx264 @ 0x1e6d1e0] i8c dc,h,v,p: 61% 18% 20%  1%
[libx264 @ 0x1e6d1e0] Weighted P-Frames: Y:2.3% UV:0.0%
[libx264 @ 0x1e6d1e0] ref P L0: 56.2% 10.7% 21.3% 11.6%  0.1%
[libx264 @ 0x1e6d1e0] ref B L0: 84.3% 11.6%  4.1%
[libx264 @ 0x1e6d1e0] ref B L1: 94.4%  5.6%
[libx264 @ 0x1e6d1e0] kb/s:569.65
$ ll Robot*
-rwxr--r-- 1 hep html 12988147 Jul 27 12:53 Robot.3gp
-rw-rw-r-- 1 hep html  1630980 Jul 27 13:43 Robot.mp4
$ file Robot.3gp
Robot.3gp: ISO Media, MPEG v4 system, 3GPP
file Robot.mp4
Robot.mp4: ISO Media, MPEG v4 system, version 1

Now the Robot.mp4 file plays in Firefox without a problem.