Check if clearing the Cache helps:

1. Go to Tools menu > Options > Advanced > Network.
2. Under the Offline Storage section, click on the Clear Now button.

Otherwise, see the possible solutions in this article: Fix common audio and video issues.

Crashes on sites with Windows Media Player video or audio

https://support.mozilla.com/en-US/kb/Firefox%20crashes%20when%20loading%20certain%20pages#w_crashes-on-sites-with-windows-media-player-video-or-audio

Thanks @dynamite and @mha007, but the problem still remains.

There were no conflicting plugins, vlc or any dlls.

go to Help Menu -> select "Restart with Add-ons Disabled"

Firefox will close then it will open up with just basic Firefox. You can enable the Add-ons later. Check and tell if its working.

You can try to disable the hardware acceleration in the Flash Player. See Flash videos won't play full screen

• http://www.youtube.com/swf_test.html (right-click the player: Settings)

Flash "Display settings" window:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/help01.html

Your above posted system details show outdated plugin(s) with known security and stability risks that you should update.

• Shockwave Flash 10.2 r153
• http://www.mozilla.com/plugincheck/

Update the Flash plugin to the latest version.

• http://kb.mozillazine.org/Flash
• http://www.adobe.com/software/flash/about/

Thanks @mha007, yes it worked for me.