If you have problems with current Shockwave Flash plugin versions then check this:

• see if there are updates for your graphics drivers

https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)

https://forums.adobe.com/message/4468493#TemporaryWorkaround

• disable hardware acceleration in the Flash plugin

https://forums.adobe.com/thread/891337 See also:

• http://kb.mozillazine.org/Flash#Troubleshooting

Separate Issue; Your System Details shows;

Installed Plug-ins

Adobe PDF Plug-In For Firefox and Netscape 11.0.0 Adobe PDF Plug-In For Firefox and Netscape 11.0.14
Having more than one version of a program may cause issues. You must remove the older programs. Then get the current full installer.

Security Issue: Update your Adobe Reader Version 2015.016.20039 Note: Other Software is offered in the download.

Do you notice any pattern to the address listed in the unresponsive script warning dialog? Does it point to the website you're on, or a different site?

FredMcD said

If you have problems with current Shockwave Flash plugin versions then check this:

• see if there are updates for your graphics drivers

https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)

https://forums.adobe.com/message/4468493#TemporaryWorkaround

• disable hardware acceleration in the Flash plugin

https://forums.adobe.com/thread/891337 See also:

• http://kb.mozillazine.org/Flash#Troubleshooting

---

Separate Issue; Your System Details shows;

Installed Plug-ins

Adobe PDF Plug-In For Firefox and Netscape 11.0.0 Adobe PDF Plug-In For Firefox and Netscape 11.0.14
Having more than one version of a program may cause issues. You must remove the older programs. Then get the current full installer.

Security Issue: Update your Adobe Reader Version 2015.016.20039 Note: Other Software is offered in the download.

All that changed was Firefox. It worked before and still works in MS Explorer. Seems to me the problem is with FIREFOX! Yes, I already did turn off the protected mode AND the accelerator. Thanks for the thoughts though!

jscher2000 said

Do you notice any pattern to the address listed in the unresponsive script warning dialog? Does it point to the website you're on, or a different site?

Well, if ALL videos are a pattern then YES. There is a pattern. Happens on You Tube and all other videos.

No, I meant when you get the Unresponsive Script dialog, what URL is listed in the dialog?

If you visit YouTube's HTML5 support page, do you have all blue boxes except the last one?

https://www.youtube.com/html5

By the way, when you posted, your browser reported to the forum that you run Windows 7, but in your description, you said this problem occurs on Windows XP. Could you clarify that? There are some important differences between the two platforms related to video in Firefox.