Separate Issue; Your System Details shows;

Installed Plug-ins

Adobe Shockwave for Director Netscape plug-in, version 12.2.9.199 Adobe Shockwave for Director Netscape plug-in, version 11.6.5.635
Having more than one version of a program may cause issues. You must remove the older programs. Then get the current full installer.

Shockwave Director: Shockwave Standalone Installer / Uninstaller{web link} Shockwave Director
http://get.adobe.com/shockwave/

This is a YouTube clip, yes? Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache and

• Remove Cookies

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

Type about:preferences<enter> in the address bar.

• Cookies; Select Privacy. Under History, select

Firefox will Use Custom Settings. Press the button on the right side called Show Cookies. Use the search bar to look for the site. Note; There may be more than one entry. Remove All of them.

• Cache; Select Advanced > Network.

[v57+] Select Privacy. Across from Cached Web Content, Press Clear Now.

If there is still a problem, 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?

While you are in Safe Mode;

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

How to disable Hardware Acceleration {web link}