Do you see any error messages in the Web Console ("3-bar" menu button or Tools -> Web Developer) about playing media files?

• https://developer.mozilla.org/Tools/Web_Console

Do you have any content blocking extensions?

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• Hold down the Shift key and left-click the Reload button
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Command + Shift + R" (Mac)

Clear the cache and remove the cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Clear the cache":

• Firefox/Options/Preferences -> Advanced -> Network -> Cached Web Content: "Clear Now"

"Remove the cookies" from websites that cause problems.

• Firefox/Options/Preferences -> Privacy -> "Use custom settings for history" -> Cookies: "Show Cookies"

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• Do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Thanks!

I tried all your advice. Behavior is the same whether in Safe Mode or normal. Without AdBlock Plus, the Console shows too many issues.

In the console screen shot, in the lines "VIDEOJS: WARN:", "index.min.js:10:2538" shows fly-over text "http://players.brightcove.net/369058143800/ByIPHnG8_default/index.min.js:10:2538".

The video in my example plays in Chrome. Its console shows the same entries. It also shows many AdBlock actions. The second screenshot is Chrome, filtered to show lines containing "media".