First, update your YouTube Center extension, which got two releases last month since the version you have installed. That might work using the built-in update check, or you may need to visit the Add-ons site:

• Built-in: orange Firefox button (or Tools menu) > Add-ons > "gear" button > Check for Updates

• Add-ons page: https://addons.mozilla.org/en-us/firefox/addon/youtube-center/ (Firefox restart needed for this update to take effect)

Does that fix it?

Although the latest RealPlayer / RealDownloader should work with current Flash players, you may want to test with those disabled. You can do that on these two tabs:

orange Firefox button (or Tools menu) > Add-ons > Plugins category
orange Firefox button (or Tools menu) > Add-ons > Extensions category

Usually a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Does that make any difference?

Other standard steps for resolving Flash issues are as follows:

(1) Disable hardware graphics acceleration in Firefox and in Flash

(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

(2) Disable protected mode (Windows Vista/7/8)

See this support article from Adobe under the heading "Last Resort": Adobe Forums: How do I troubleshoot Flash Player's protected mode for Firefox?

Updating Youtube Center seems to have solved the problem, at least for now. Thank you!

In the upper left corner of the URL link bar - there is a little gray shield.

LEFT CLICK IT and a drop down menu will appear - click on the selection that is marked KEEP BLOCKING - and choose - DISABLE PROTECTION ON THIS PAGE.

The videos should work now. It's a security feature that must be manually set by the user.

Hope it works!