Does this happen with all WebM videos or only on specific websites?

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do not click the Reset button on the Safe mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Also make sure that your security software (firewall, anti-virus) isn't blocking the videos.

Please furnish a link to any WEBM video that doesn't play properly for you in Firefox.

For example, does the embedded webm video at http://www.mozilla.org/firefox/video/ play OK?

What about this link? http://videos-cdn.mozilla.net/brand/Mozilla_Firefox_Manifesto_v0.2_640.webm

What about the embedded webm video at http://camendesign.com/code/video_for_everybody/test.html

What about this link? http://clips.vorwaerts-gmbh.de/big_buck_bunny.webm

After a bit of extra troubleshooting, I found out that the problem was something completely unrelated to Firefox and was, instead, a problem with the video I was trying to play. Thanks for your help, though; I really appreciate it!

You're welcome

I am using Firefox 22.o currently on beta channel. I am using Windows 7 64 bit

I am trying to play the following URL (to see whether webM is working or not)

http://www.flumotion.com/demosite/webm/

Getting an error as Video format or MIME type not supported

Any pointers will be appreciated

rktenneti, The video that is supposed to be on that demosite should be at http://195.10.10.210/flumotion/videodemo/high.webm

However, that link results in an error,

No Such Resource
File not found.

Contact the webmaster. If you need more help you should ask a new question since this one is solved (problem with the video).