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Firefox cannot play mp4 files on windows 7

  • 11 replies
  • 36 have this problem
  • Last reply by nancyjahn

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I'm using firefox 30 on Windows 7. I was under the impression Firefox supported mp4 playback in this configuration as described at but I can't get it to work anywhere.

webm and ogg theora work fine but mp4 playback doesn't. Some sites I've tested: - top video does not load - left side videos do not load - video playback fails completley youtube with html5 player- video playback fails completely

Does anyone have any solutions?

All Replies (11)

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Do you have a Windows version that comes with Windows Media Foundation and this enabled?

You can also check these prefs on the about:config page.

  • media.directshow.enabled
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thanks for your response

I am using Windows 7 Ultimate which has windows media foundation to my knowledge.

both and media.directshow.enabled are set yo true

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You can open the Web Console (Firefox/Tools > Web Developer) and check for issues with loaded media files.

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To rule out extensions as a possible culprit, could you try the test pages in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions, some advanced features, and some custom settings. More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using either

  • "3-bar" menu button > "?" button > Restart with Add-ons Disabled
  • (menu bar) Help menu > Restart with Add-ons Disabled

Note: Flash and other plugins still run

After Firefox closes, a small dialog should appear: click "Start in Safe Mode" (not Reset)

Any difference?

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With the last Firefox upgrade, some plugins are no longer enabled until you enable it.

Open the Add-ons Manager. Press the <Alt> or <F10> key to bring up the tool bar. Followed by;

Tools' Add-ons.

Hot key; <Control> (Mac: <Command>) <Shift> A)

On the left side of the page, select Plugins.

BTW; At the top right is a link to check your plugins.

Modified by James

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In the console I see this for each video: Media resource <file path> could not be decoded.

Starting in safe mode gives the same result. I've tried enabling/disabling all browser plugins and also disabling plugin scanning but I get the same error regardless.

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Are you using any kind of blocking software in Firefox or your computer?

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No blocking software, all addons/plugins disabled and still the same problem.

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It is likely that you do not have the codec to play such a file via the HTML5 media player.

Does Firefox use another player if you disable the HTML5 media player?

As a test you can disable the HTML5 media player by toggling these prefs to false on the about:config page.

  • media.directshow.enabled
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Firefox uses flash when those settings are disabled.

Is there a way to install the codec? I thought Firefox could play mp4 on Windows 7.

Modified by homunyan

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Maybe your mp4 file has some problem. You can try another mp4 file to have a test. Change your mp4 file to webm also is fine to have a try.