firefox - this browser does not support video playback

I have problem watching twitter embedded videos. I got this error message: "This browser does not support video playback."

Here is an example

I have Firefox version 43.0.

You should -

1. make sure that gstreamer1.0-libav plugin is installed

2. then in firefox address bar go about:config, search mediasource & enable media.mediasource.webm.enabled & media.mediasource.enabled

That should allow playback on twitter as long as the 1st. 2 mentioned below are not enabled or you enable all 3

If you want full coverage in Firefox as seen in https://www.youtube.com/html5, (all 6), then in about:config in addition to above search media & enable media.fragmented-mp4.exposed & media.fragmented-mp4.ffmpeg.enabled & needed to not break facebook media.fragmented-mp4.gmp.enabled

As they say proof is in the pudding -

You can use Pepper Flash in firefox. Pepper Flash is basically Chrome's Flash API. Chrome has this API installed by default (You might notice this when you have a fresh install of Windows/Linux and Chrome doesn't need to install flash while other browsers do).

Here are the steps:

1. Open terminal by clicking the Ubuntu logo, the Super key or press Ctrl+Alt+T
2. Type or copy in

sudo add-apt-repository ppa:nilarimogard/webupd8
sudo apt-get update
sudo apt-get install freshplayerplugin

3. Install google-chrome from it's website or use the chromium way in the link below (The reason you install Google Chrome is for the freshplayerplugin to recognize the Pepper Flash in Chrome and load it into Firefox)

Open firefox and go to about:plugins. This is what you should get (notice the flash version):

Uninstall Linux Flash by doing

sudo apt-get purge flashplugin-installer adobe-flashplugin adobe-flash-properties-gtk adobe-flash-properties-kde

Source: Can I use Chrome's Pepper Flash with Firefox?

I have just updated Firefox to version 43.0 and now the video is supported and playable.