hi, please see if How to troubleshoot security error codes on secure websites can help.

Thanks for sharing the article. In my case the 'Add Exception' button never shows up! What can I do?

as it is stated in the article, you should not add an exception for such major sites, but get the root cause of the issue fixed. which security software are you using?

Extremely sorry for the delayed response. It's in my office PC . But I'm able to browse youtube through other browsers without any issue...

I think Philipp may still be interested to know what security software is used.

Additional info that may also help, as the PC is used in an Office

• Have you tried your Company IT department for help & advice ?
  What did they say ?
• Is this computer used on different Networks, and is it only a specific Network you have issues with: Only at Home or only in the Office for instance ?
• Is it only YouTube that is ever a problem ?
  So for instance when you see the YouTube problem on Firefox you are still able to connect to our support site OK ?

Thanks for the response. - Since I'm able to browse youtube through another browsers I have not informed IT team about this issue.

- McAfee is been used

- Till now no other websites showed up this issue.

Sincere thanks to everyone who listened and responded.

After updating to FireFox 44.0.2 I'm able to add exception and browse youtube.com now.

Thanks to all.