What is the error message? Is the entire page missing, or is the problem in the Flash player (black rectangle)?

More generally, when you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Bypass Firefox's Cache

Use Ctrl+Shift+r to reload the page fresh from the server.

Alternately, you also can clear Firefox's cache completely using:

orange Firefox button (or Tools menu) > Options > Advanced

On the Network mini-tab > Cached Web Content : "Clear Now"

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, try either:

• right-click and choose View Page Info > Security > "View Cookies"
• Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies"

In the dialog that opens, you can remove the site's cookies individually.

Then try reloading the page. Does that help?

Hi, I Cleared Cached Web Content,Cookies. Still Same Issue.

If you only have problems with Flash on YouTube, it could be something specific to YouTube. For example, some ad blocking and privacy protecting extensions need exceptions for YouTube in order to play videos normally.

You can check your extensions and see whether any of that might be relevant, or test the site in Firefox's Safe Mode to see whether ANY extensions are the problem. More info on Safe Mode: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled (Flash will still run)

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference?

Two other YouTube specific comments:

(1) You can play many videos in Firefox's native HTML5 player instead of Flash and see whether that helps.

https://www.youtube.com/html5

(2) Some users have reported that this error is associated with the DASH method of sending videos in small bits and use the YouTube Center extension to revert to the older method of sending the whole video as one giant file.

https://addons.mozilla.org/en-us/firefox/addon/youtube-center/

And more generally, this is some standard guidance that addresses the most common issues with the Flash Player plugin. I'm sure you've seen some of it before, but just in case:

(1) If you have any recorders/downloaders that interact with Flash media make sure they are as up-to-date as possible, or disable them temporarily.

(2) 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

(3) Disable protected mode (Windows Vista and higher)

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

Any luck?