Hi there!

The problems associated with your issue could be:

- NO flash enabled or installed.
- PC memory is too slow because of which the whole process stalls.
- Bowser cache is full.

Try to check these issues, if the problem persist, please feel free to inform us.

To rule out a conflicting extension or graphics driver problem, could you test YouTube in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

Also, are you allowing the new "HTML5" player on YouTube or are you forcing the traditional Flash player? To see which player you are getting, assuming you can run YouTube for at least several seconds, right-click the player and check the last line of the short context menu that appears. If Firefox's long context menu appears, press and release the Alt menu to remove it so you can see the site's menu.

Eventually firefox (Not Responding) posts a pop-up window with: "A script on this page may be busy, or it may have stopped responding. You can stop the script now, open the script in the debugger, or let the script continue. Script: https://s.ytimg.com/yts/jsbin/…S-vflOWWv0e/html5player-new.js:473" So why does firefox (Not Responding) still have a problem with HTML5?

On one occasion during the firefox (Not Responding), a Windows pop-up reports an error of plugin-container.exe

IE 11 does NOT have this problem. It is just plain slow, but it plays nice enough with HTML5.

I haven't seen any unresponsive script errors with the HTML5 player yet myself, but complex scripts sometimes do run slowly. If you click continue, does the problem resolve or repeat?

A crash in plugin-container.exe is, as you would expect, caused by a problem with a plugin. Any idea which one the page was using? Since the most commonly used plugin is Flash, I'm inclined to suggest this standard guidance. I'm sure you've seen some of it before, but just in case:

(1) If you have any recorders/downloaders that interact with Flash media make sure they are as up-to-date as possible, or disable them temporarily.

(2) Disable hardware acceleration in Flash: right-click the media in the player and choose Settings, then the first mini-tab and uncheck the box to use hardware acceleration.

More information in this support article from Adobe: http://helpx.adobe.com/flash-player/kb/video-playback-issues.html#main_Solve_video_playback_issues

(3) Disable protected mode (Windows Vista and higher)

The protected mode feature of the Flash player plugin has security benefits, but seems to have compatibility issues on some systems. You can disable it using the Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. On the right side, find "Shockwave Flash" and click the More link. Then uncheck the box for "Enable Adobe Flash protected mode" and try that for a day to see whether it helps.