This browser does not support video playback.
I've gotten used to switching to a different browser when the above message appears on videos posted on Twitter. Every now and again, though, I get frustrated and spent absolutely ages trying to find a reliable way to get the damn things to play in Firefox or the Firefox-derived Pale Moon, which is my current default browser, but I've never yet had any luck. And the sheer number of search-results leading to other people's attempts to solve the same problem convinces me that it's an extremely common problem.
Proposed solutions generally include all the stuff most of us have already tried before resorting to searching message-boards for answers. Updating flash and enabling or disabling flash or HTML5. Then there's suggestions regarding changing various entries in the about:config page. Some of the discussion gets esoteric to say the least and none of it seems to work. Either you have the problem or you don't, and if you do, it seems that by and large you're stuck with it. So back to opening a second browser then.