That's inconvenient. If you right-click the video, does "Show Controls" appear on the menu? Usually it says "Hide Controls" but if the controls are hidden using a standard method, then you should have "Show Controls".

If that doesn't work, can you give a link to an example page where the video has this problem?

Thank you for taking the time to help me. Fortunately I solved it though. If I right clicked like you suggested I got an option to play, mute, set speed, or watch in full screen. There was no hide/show controls. The issue was for ALL .webm files, perhaps all videos. The problem turned out to be my addon, Policeman, or rather, the problem was me. Policeman is a whitelisting tool blocking 3rd party content that are not allowed. I never understood the Chrome destination on many websites so I almost never allowed these for the websites I used.. This time I did and now it works.

Glad to hear it is solved, but I'm not sure I understand "the Chrome destination on many websites." Firefox uses chrome:// as the protocol for some internal pages (including some add-ons settings pages), so if those are popping up when you get to pages with WebM videos, perhaps they are associated with a different add-on?