What's the link to the page with the video? The error message suggests it's a page that tries to play an HTML5 video, not a Flash video. Examples:

• http://ie.microsoft.com/testdrive/graphics/videoformatsupport/default.html
• http://www.quirksmode.org/html5/tests/video.html

Install the Windows Media Player Plugin for Firefox

• http://www.interoperabilitybridges.com/html5-extension-for-wmp-plugin

Hi, i installed the Windows Media Player Plugin for Firefox but it's still not working.

The format of video player is not supported by firefox. Try this to resolve it

• https://developer.mozilla.org/en-US/docs/HTML/Supported_media_formats
• https://developer.mozilla.org/en-US/docs/Using_HTML5_audio_and_video
• https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Using_HTML5_audio_and_video

You can also try the VLC Plugin provided by the VLC Media Player.

Download from here http://www.videolan.org/index.html

Note that the Windows media plugin has been unsupported for a long time so you should uninstall that.

Will you not post the link to the video like I requested, or at least let us know if you can play the videos on the pages I linked to? It would prevent more random guesswork.

Firefox doesn't try to play files it can't handle. When coming across a direct link to a file that could be handled by a plug-in, but that plug-in isn't installed, Firefox would display a download prompt.
That's not the issue here, where there's an unsupported format error. That suggests an MP4 file that can't be played. Even if a plug-in that can handle MP4 files were installed, Firefox probably wouldn't use it due to bug 861090.

Firefox has supported HTML5 H.264 video on Windows Vista since version 22.0. There's no point in installing the very old — and likely abandoned — Windows Media Player plug-in.

You can try as a possible workaround to right-click the video player and left-click or middle-click the "View Video" item in the context menu to open the media file in a tab.