When I go to a certain web page (have only noticed this on this particular site), the place where a video should be is blue. Works in Edge.

If you have visited the site previously without issue, try clearing cookies and site data and clearing the cache. https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

It's Malwarebytes. I don't like disabling it permanently, unfortunately.