Check this it may help you

https://support.mozilla.org/en-US/kb/how-do-i-watch-flash-videos-firefox-android

https://support.mozilla.org/en-US/kb/why-cant-firefox-mobile-play-flash-on-my-device

better update your flash plugins

The video is still not working and it is not a flash video. I think it is mp4 video and the same with sound. Other browser do not have this problem. And it is not a settings thing (like "click to play").

Which specific video. Which specific Phone. Support for h264 playback is phone dependent.

I've also found that some videos do not play (or do not play correctly). Nexus 5 running latest Kitkat.

Where there is a link to the video, I can long-press it and launch it in an app (video player) as a workaround. Sometimes if I long press the video itself (the black area that isn't playing) I get a copy link or share option and if I paste the link in the address bar, then I can launch it in an app. In other cases, I don't get a menu and I'm stuck.

Since it is hard to see under the covers and see what each page is doing, it is hard to figure out the pattern...

The video that I can't play is on a Swedish newspaper (Expressen) and on viasat. They sometimes remove video but here is a link to the videos and no video is playing.

http://www.viasatsport.se/videoklipp/

and here is the videos on Expressen: http://www.expressen.se/tv/

My phone is a Motorola Deffy+ (android 2.3.6) and the videos is working fine on some other browsers (android browser, opera). And it is not working on my tablet (android 4.0.4) either.

But it is not only video but also sound. Here is a homepage that display sports result. When some team makes a goal a sound is playing if live game is selected. http://www.liveresultat.nu

And here is a Swedish radio that is not working. I can't listen to non of the station they have. http://sverigesradio.se/