This doesn't appear to any message that would come from the Firefox browser itself. According to your system details, it looks like you have a lot of add-ons installed on Firefox. It's most likely one of them that's creating this message, although I don't see any obvious one that would be causing the problem. The best way to check if it's an add-on issue is to [[Troubleshoot Firefox issues using Safe Mode|run in Firefox Safe Mode]] for a little bit to see if the message appears. This mode temporarily disables your add-ons. If the message doesn't appear when in Safe Mode, you'd have to find out which add-on is causing the issue via trial and error by disabling them one at a time. It's not uncommon for video downloading add-ons to have this type of behaviour, although there's no add-on name that pops out at me when I looked at the list. Hope this helps.