Hello, can you provide more description around the problem. Such as how far you get with the video. If the video loads and plays. Or if the video has any disruptions while playing. Etc. Thanks!

See also:

• https://support.mozilla.org/en-US/kb/fix-video-audio-problems-firefox-windows
• https://support.mozilla.org/en-US/kb/fix-common-audio-and-video-issues
• https://support.mozilla.org/en-US/kb/html5-audio-and-video-firefox

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?

Zachary_ said

Hello, can you provide more description around the problem. Such as how far you get with the video. If the video loads and plays. Or if the video has any disruptions while playing. Etc. Thanks!

When the video is supposed to play all I get is a black screen.

Zachary_ said

Hello, can you provide more description around the problem. Such as how far you get with the video. If the video loads and plays. Or if the video has any disruptions while playing. Etc. Thanks!

When the video is supposed to play all I get is a black screen. FredMcD said

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?

Video plays in safe mode. What does this tell me?

It could be the work of one of your add-ons. 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.

Your System Details shows;

Installed Plug-ins

YouTube Flash Video Player 54.0 ({f3bd3dd2-2888-44c5-91a2-2caeb33fb898}) YouTube™ Flash® Player 2.0.2 (jid1-HAV2inXAnQPIeA@jetpack)

These may be the issue.

I did what you suggested and disabled one at a time. There was an HTML5 Video Everywhere that seems to be the one causing the problem. With it disabled YouTube videos play fine. Thanks for your help Ray G

Good to hear. Contact their support.

I get an error message in the black window for every YouTube video now saying "Your browser does not currently recognize any of the video formats available. Click here to visit our frequently asked questions about HTML5 video." I don't know what the problem is, since YouTube worked fine with Firefox for Mac, my browser, in the past and is fine with Chrome as someone else mentions here. I have the current 55.0.3 version, so what's wrong here?

I have had one or two ad blockers in my Firefox browser for quite awhile with no problems in playing YouTube videos. I would prefer to keep them active and wonder why, if I've added no new add-ons for awhile, why would YouTube suddenly have any issues because of these.

I see you have several YouTube add-ons. Disable them.

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?

Raytgal said

Lately I have been unable to play some videos, youtube, vimeo. I can play them on Chrome. Today I removed firefox and downloaded a fresh install. No help. Same problem. Help please?

I am not sure but the Firefox install may leave some of the settings from the old installation in the configuration file. I have no proof of this but the suggestion of FredMcD at post 3 or 4 will show a list of what is active in your installation

tjphoto40 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. 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.

Hi Guys,

I have been miserable with a issue where YouTube Videos says that the player in my browser is not on a list of supported players. When I ran across the question started here I Read it down to the reply from FredMcD.

Here is the part that helped me:< In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

This page showed that I have a legacy YouTube Flash Player extension. There was a options button!!!! I pressed the button and there it was, A selection box popped up and it was titled, YouTube Video Player. the selection window showed, Flash Video Player. There was a selection arrow button to the right side. I clicked it and there it was, HTML5 Video Player. This legacy YouTube Flash Player extension was forcing all YouTube videos to play using Flash. When I restarted the page with the video, BAMM there it was. I have checked this with several YouTube pages that did not play and they are all good!!!

That was very good work. Well done.

Legacy just means that in a future version, those add-oms will not work in Firefox. They will need to be rewritten or replaced.

FredMcD said

I see you have several YouTube add-ons. Disable them. 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?

Before I received this and another reply that seemed directed to my posting, I noticed a little icon in the upper right of the Firefox window, next to the menu icon, that is a Flash Player button that can be turned on or off. (Is this what's meant by a Legacy version of Flash?) I simply clicked on it to turn it off, and the YouTube videos all appeared normal again. So I didn't need to do anything more than that. Some of the other methods suggested here struck me as overly complex or questionable, and I was worried about harming some other aspect of my browser function. But at least I figured out this simple fix. Thanks.

Good Work.