I think I've solved a lingering problem I've had playing certain videos in Firefox, and I'm posting the details here mostly so I don't forget them and partly in case anybody else is tripping over this.
I will periodically land on a web page with a video at the top. Usually I don't want to play the video, but sometimes I do ... in which case I have mixed results getting the video to play in Firefox. Typically Chrome has no such problem with the page, which makes sense given that the issue in Firefox relates to a couple of browser extensions I use. What made diagnosing the problem tricky was that (a) results were inconsistent (some videos played, some didn't) and (b) the problem resulted from a combination of two extensions, not just one particular one.
They symptom was that, when I clicked the play button on a video, the player box would turn into a black screen with a spinner that would spin indefinitely, until an error message popped up saying that the video player had hit a time limit waiting for the video to load. Again, this happened on some videos but not others. In particular, I never had a problem with a YouTube video. It was at least consistent in that a video that triggered the error would always trigger the error, regardless of page reloads etc.
The two Firefox extensions involved are HTTPS Everywhere (which tries to force page contents to load using the more secure HTTPS protocol than the ordinary HTTP protocol) and Disable HTML5 Autoplay (which prevents videos from automatically starting to play). The first extension is a security enhancement. (For an opinion piece on why HTTPS is important, read "Why HTTPS Matters".) Unfortunately, many web sites either do not deploy HTTPS for some content or screw up their site configuration. Regarding the second extension, I find video autoplay to be rather annoying, since it frequently involves ads or other videos that I have no interest in, and forces me to play whack-a-mole to shut them the bleep up.
I'm loathe to give up either extension, and fortunately I don't have to. What works for me is a combination of two tweaks. On a site where I get problem videos (time.com is the main source, in my case), I click the toolbar button for the autoplay extension, leave "Disable Autoplay" selected, but deselect "Disable Preloading". That only needs to be done once per site. With a page open containing a problem video, I then disable HTTPS Everywhere (again, by clicking its toolbar button and deselecting the first option). That should automatically cause the page to reload, and the video will play properly. After I'm done watching, I just reenable HTTPS Everywhere. This part has to be repeated for each page containing a video that will not load via HTTPS, but it's a price I'm willing to pay to preserve security.