Are you using AdBlock Plus add-on ??

1. IF No, then see -> Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems
2. IF Yes, then click the round Red ABP button of AdBlock Plus
   • from the sub-menu, click Disable on youtube.com
   • Restart Firefox
   • go to youtube.com

Check and tell if its working.

You can use these steps to check if images are blocked:

• Open the web page that has the images missing in a browser tab.
• Click the website favicon (How do I tell if my connection to a website is secure?) on the left end of the location bar.

• Click the "More Information" button to open the "Page Info" window with the Security tab selected (also accessible via "Tools > Page Info").
• Go to the Media tab of the "Tools > Page Info" window.

• Select the first image link and scroll down through the list with the Down arrow key.
• If an image in the list is grayed and there is a check-mark in the box "Block Images from..." then remove that mark to unblock the images from that domain.

See also:

• http://kb.mozillazine.org/Images_or_animations_do_not_load
• https://support.mozilla.org/kb/Images+or+animations+do+not+show

my problem is solved. Thank you for your help.