If they are family-friendly websites, do you want to post an address or two?

You also could investigate a little further by right-clicking the video in Internet Explorer and seeing whether the context menu that appears lists the particular plugin used to play it (for example, the bottom line of a plugin menu often gives its name or offers more information about it).

In case you need the Windows Media Player for those sites, please see this article for where to get the plugin, and a setting or file location that needs to be changed to compensate for a change in Firefox since the plugin was posted: Play Windows Media files in Firefox with the Windows Media plugin.

I suppose I also should mention that some popular add-ons, such as ad blockers and privacy protectors, may prevent Firefox from loading some parts of pages. If the videos play in Firefox's Safe Mode, then you probably need to adjust the settings of those add-ons.

Firefox's Safe Modeis a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. Here's how you can try it:

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

More info: Diagnose Firefox issues using Troubleshoot Mode

To start up in normal mode again, exit out of Firefox and when you start it up again, don't hold down the Shift key.

I have found the same issue. But I discovered something curious. My download managers sometimes can detect the video even if there is no player on the screen.