Hello,

Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• You can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
• Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Once you get the pop-up, just select "'Start in Safe Mode"

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one. Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

To exit the 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 other users who have the same problem.

Thank you.

In SafeMode problem still exist.

Recent crashes of certain multimedia contents (this includes Youtube videos, certain flash games and other applications) in conjunction with Firefox are most probably caused by a recent Flash 11.3 update and/or a malfunctioning Real Player browser plugin.

In order to remedy the problem, please perform the steps mentioned in these Knowledge Base articles:

Flash Plugin - Keep it up to date and troubleshoot problems

Adobe Flash plugin has crashed - Prevent it from happening again

Flash 11.3 doesn't load video in Firefox

Other, more technical information about these issues can be found under these Links:

http://forums.adobe.com/thread/1018071?tstart=0

http://blogs.adobe.com/asset/2012/06/inside-flash-player-protected-mode-for-firefox.html

Please tell us if this helped!

I'v tried to downgrade flash from 11.3 to 10.3 - no effect. I have no real player plugin at all. Flash player dose NOT crash it just don't load youtube videos in FFox. Also I'v use wireshark and it shows, that youtube return "HTTP/1.1 204 No Content" when it try to download video

Try uninstalling anything flash related then reinstalling, has this just begun happening or did this happen after a update?

It started after avast base update in all browsers. Then I've uninstalled avast and all browsers start working correct, except FFox. Also I've asked other ppl, who use other anti-virus software and they have same troubles in FFox on youtube. If it would help I'll ask them if they update flash player before. But any way it does not explain why downgrade to 10.3 have no effect.

Try creating a whole new profile then see if the problem persists. https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

No effect =(

How about trying it on another browser such as Google Chrome or Internet Exploder, does that work, if so do below.

Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove Firefox program files and then reinstall Firefox. Please follow these steps:

Note: You might want to print these steps or view them in another browser such as Google Chrome or Internet Exploder.

1. Download the latest Desktop version of Firefox from http://www.mozilla.org and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (click Exit from the Firefox or File menu).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

Please report back to see if this helped you!

Same story. Even Clean reinstall has no effect. So it can be only flash player trouble? But why only in FFox?

The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information. Note: This will cause you to lose any Extensions, Open websites, and some Preferences.

To Reset Firefox do the following:

1. Go to Firefox > Help > Troubleshooting Information.
2. Click the "Reset Firefox" button.
3. Firefox will close and reset. After Firefox is done, it will show a window with the information that is imported. Click Finish.
4. Firefox will open with all factory defaults applied.

Further information can be found in the Refresh Firefox - reset add-ons and settings article.

Did this fix your problems? Please report back to us!

Thank you.

Nope. YouTube still wait about one minut before loading video, then load 10 sec of video and wait again. Even video advertising cant be loaded.

Also html5 player works with lightning speed, but youtube use it only in 30% of videos.

Download Firefox Portable and see if flash works there.

In portable all works fine. And, thats strangely, after run portable version, full version begin to work correct too. So I may say that there is no problem any more. But what was it?

I have no idea what it could had been, please notify us if the issues come back.