It does help if there is a site in reference as this general statement doesn't provide how or why for anyone trying to find out what is the cause.

No site will play. Youtube. Fox news. The Blaze. None. The video appears to load. I can even see the thumbnails along the playback line. But the video will not play.

Netflix is not playing on my computer.

Make sure you are not blocking content.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

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

Ok, the videos run in safe mode but not if I just run Firefox normally. What do I do to stop blocking content?

Ok, the videos run in safe mode but not if I just run Firefox normally. What do I do to stop blocking content?

It could be the work of one of your add-ons, or it could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All Of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is, detective.

Disabling addons made no difference but when I turned off hardware acceleration all of the videos came back to life. Thank you so much for your help.

Glad to help. Safe Surfing.

Videos do not load or play on any web site. I get just a black area where the video should be. I tried restarting in safe mode and that does not solve the problem.

None of my browsers--Firefox, Chrome, and Safari--will play videos, with a few exceptions. This problem started suddenly, about 30 minutes ago. The video at http://www.labastidedemarie.com/#!villa-luberon/sejour-luberon-menerbes played then, but not now. Nor do videos at the New York Times, YouTube, Daily Motion, or other websites. However, weather radar will play at http://www.intellicast.com/National/Radar/Current.aspx?location=USFL0438&animate=true and http://www.baynews9.com/fl/tampa/weather/radar. The video at https://www.apple.com/iphone-x/ seems to play.

Videos I have already downloaded play in the VLC player.

I have not changed any settings.

I have run a Malwarebytes scan, which came up clean. I updated Flash. I cleared the caches for each browser. I unchecked "Use hardware acceleration when available" for Firefox. I emptied the system and browser caches. I ran Disk Repair. I shut down and restarted. Nothing works.

iMac 27-inch Mac OS 10.13.6 Firefox "About Firefox" stopped working just now. It is not Quantum. I see the usual window briefly, for less than 1 second, then nothing but a white screen. Safari 11.1.2 Malwarebytes free version

Garak and spyhop

If the above did not help, try this.

Make sure you are not blocking content.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

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 Tracking Protection feature. 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: Tracking Protection {web link}

But if there is still a problem, Please go to the top of any support.mozilla.org web page and use the Ask A Question link; https://support.mozilla.org/en-US/questions/new Select the product for what the problem is (Firefox or whatever).

Now select the category for the problem. At the bottom is Other.

Describe the problem in brief (one line). Then press <enter>. The website will display any prior posts that might help you. If nothing on the list can help, at the bottom press the button; None Of These Solve My Problem.

Now enter all the relative information on the problem. At the bottom of this, you can Add Images.

Under this is Troubleshooting Information. Please press the Share Data button. This will let us look at your system details. No Personal Information Is Collected.

Thanks. I rebooted my iMac, and things now seem to working properly. I am still puzzled as to what could have caused such a system-wide failure.