Flash Player Version 18.0.0.232
https://www.adobe.com/products/flashplayer/distribution3.html Note: Windows users must download the “Internet Explorer” and “Plugin-based browsers” installers.

Note: Edge on Win10 does not use ActiveX version but its own Flash.

See if there are updates for your graphics drivers https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

I spent a minute on that site and it seems to be set up for downloads instead of online viewing, at least for the one show I looked at.

In the Question Details > More System Information next to your post, the Flash player plugin seems to be missing. It might be blocked as being very out-of-date.

You can get version 18.0.0.232 on this page:

https://www.adobe.com/products/flashplayer/distribution3.html

In the first table, look for the row for "plugin-based browsers" and you can use either the EXE or the MSI installer. Or you could try the "Extended Support Release" of Flash 13, which is further down the page and might be better suited to Windows XP.

After running the installer, open Firefox's Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. On the right side, look for "Shockwave Flash", particularly toward the bottom, and if necessary, change "Never Activate to either "Always Activate" to "Ask to Activate".

If you cannot activate Flash, click the "More" link for the plugin to check that it detected the updated version. If not, you might need to exit Firefox and start it up again.

Success?

Does that make any difference for the videos that don't play?

By the way, if you need a new video to watch, I really like this one: https://vimeo.com/97455734