I am sorry to hear that there is a hissing noise when you watch videos on facebook. If this does also happen on other sites that play videos please consider that the speaker may need to be replaced?

First try this: What to do if Firefox won't play any sounds

YouTube normally uses the HTML5 media player and not the Flash plugin in current Firefox versions. You can check that via the right-click context menu in the player area. The Flash player always shows the Flash version that is used. You can open the YouTube test page via "About the HTML5 player".

• https://www.youtube.com/html5

Make sure you have the latest GStreamer updates. You can see if it helps f you disable GStreamer support by setting this pref to false on the about:config page.

• media.gstreamer.enabled

Hi thanks for ur reply.. youtube video and audio is fine.. i am having issues in playing only facebook videos.. i tried ur suggestion of disabling media.gstreamer.enabled but it doesnt help.. any other suggestions ?

The clip itself might be damaged.

Hi.. i am saying all the facebook videos from fb server is not playing only in mozilla.. how can it be playing in chrome n not in mozilla ?

Can you provide a public link?

i am using firefox version 43.0.4 in Ubuntu 12.04 Linux..U mean i should give u my fb credentials ?

FredMcD said

Can you provide a public link?

what do u mean by public link ?

No login or anything.