Hey, I had the same issue on Firefox with several plugins, but it turned out it was not a plugin that was the cause but a setting in my browser.
In the settings, in the “Privacy & Security” tab, I had the “Enhanced Tracking Protection” set to “Strict”. It thinks the video player is a tracker and blocks it for some reason. I saw it in the dev tool of Firefox (when you press F12).
You can leave this setting as “strict” and disable it only for GameDev.tv by clicking on the shield at the left of your address bar.
Hope it will help!
Source: Enhanced Tracking Protection in Firefox for desktop | Firefox Help