The screen around the video is black, or the video area is black?

Sometimes incompatibilities with your graphics card drivers can cause strange display artifacts. Here are two places where you can change a setting to try to rule that out.

(A) In Firefox, un-check the box here and restart:

orange Firefox button (or Tools menu) > Options > Advanced > General > "Use hardware acceleration when available"

(B) In Flash, see this support article from Adobe: http://helpx.adobe.com/flash-player/kb/video-playback-issues.html#main_Solve_video_playback_issues

Any difference?

jscher2000: I was unable to resolve my issue by implementing suggestions (A) & (B) in your post. However, suggestion "(B)"did help me. I reviewed the support article you suggested. Because of the article I decided to install 6 recommended windows updates. I also enabled a few disabled Add-ons in Firefox. The problem was caused by disabled real-player, and flash-player Add-ons. YouTube videos are now visible and playing well. I appreciate your prompt reply to my post! Thank you!!