Is this with many websites, some, a few?

Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

One issue that seems more common lately is Firefox's Content Blocking. When it is blocking content in a page, a shield icon will appear at the left end of the address bar next to the padlock icon. This article has more info on managing this feature:

https://support.mozilla.org/en-US/kb/content-blocking

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Firefox shows the shield icon at the left end of the location/address bar in case Content Blocking is blocking content.

• click the shield icon for more details and possibly disable the protection

You can check the Web Console about what content is blocked

• "3-bar" menu button or Tools -> Web Developer
• https://support.mozilla.org/en-US/kb/content-blocking

Thanks FredMcD and cor-el

Try this on.

Combining both suggestions, I started FireFox in safe mode at the particular website that had this problem. Worked fine, no problems.

So I went through my add-ons/extensions and disabled all that might have an impact and restarted FireFox as normal. Worked fine, no problems.

So I thought, obviously one of the disabled extensions must be creating the problem with JW Player.

So I went through and enabled 1 extension at a time, shut FireFox, and then reopened the problem website. over several openings, all worked fine, no problems.

I finished with all extensions enabled and .... Worked fine, no problems.

The only thing I noticed was an extension that FireFox had disabled (see attached) , that I can not enable or delete.

Thanks to each of you, but I do not understand why disabling and then enabling the same extensions resolved the problem. Strange.

Press 'More Options' and try to update the add-on. And/or press the 'More Information' button.

FredMcD

"More Options" only give me screen shot "disabled 2" with no update available.

"More Information" is unhelpful, see "Blocked"

Where did you get that add-on? Have you contacted their support?

kenlcarter said

I finished with all extensions enabled and .... Worked fine, no problems. ... I do not understand why disabling and then enabling the same extensions resolved the problem. Strange.

If I had to guess:

Firefox cached a file during your testing that was being blocked by one of your content/ad/tracker-blocking extensions (e.g., Adblock Plus, Privacy Badger, or uBlock Origin). So currently, Firefox can run JWPlayer because it has that file at hand and doesn't need to download it again, but the problem may well return when the cached file expires out of the cache.