You may have Firefox Flash Protected Mode enabled.

Protected Mode is designed to make it more difficult for hackers to attack your computer.

You can disable it but it makes your computer more vulnerable!

You do not need Flash Player for YouTube; Firefox will automatically play in HTML5 mode.

Adobe Flash protected mode in Firefox

no, protect mode was disabled. Must use fp, because html5 not for my old pc (win xp, 1.6ghz, 1gb ddr1) :D

Hi, this might be caused by one of your extensions: Add-ons Manager (Ctrl+Shift+A > Extensions), so try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem. Thank you!

I think I found a suspected problem, but no solution. I have to delete plugin-container .exe from the C: /Program Files / mozila ... because it takes up too much cpu. Without it does not work, and when you do not delete plugin-container youtube video work, but container take on 90-95% cpu

FlashPlayer is now only supported running in the plugin-container. That it uses 90-95% of the pc cpu is probably not surprising. Where possible try to use lower resolution or lower frame rate video. Sometimes I found rather non-intuitivly that video plays better full screen than in a reduced size. Possibly the fullscreen is the native resolution and smaller frames involve additional processing.

I have had low spec XP PCs that used to play flash Player ok, at least on lower resolutions. I still have an XP laptop but IIRC it is dual core and 2GB so not directly comparable to your PC.

I am not sure HTML5 uses more resources or is not compatible with an old XP. My XP laptop will play HTML5.

html5 is compatible, but video choppy with html5

If you wish to force FlashPlayer on YouTube try one of these addons

• Search link

oops, fixed link

Thank you, it's ok for youtube. But, have problem with facebook games now. Thats extensions are for YT only

  :D

As FlashPlayer is working now even on YouTube maybe it helps if you start a fresh question about the Facebook games not working. That is something I do not use and have no knowledge about.