It could be a security issue with the embedded page. Could you try installing from the Add-ons website and see whether that works any better.

https://addons.mozilla.org/firefox/

Yes sir you are right it's a security issue.

When I click on link you provided. it shows me page without loading css and js. And when i click on download it will show your connection is not secured. So for that i uninstall my antivirus and also check that my date and time is upto date but both of it not solving my problem.

Are you able to load the CSS files using a direct link? For example, this is one of the external style sheets used on the Add-ons site:

https://addons.cdn.mozilla.net/static/css/tabzilla/tabzilla.css

Are you having a more general issue with HTTPS sites? If you have been getting similar errors on many secure sites, please pause and let's investigate the source of the problem. If your system clock is correct, it's nearly always a "man in the middle" presenting Firefox with fake certificates for sites you visit. That could be your security software, proxy server, parental control, or malware/spyware.

This article may help flag up the source but, if not, I'll suggest some options for further investigation: How to troubleshoot security error codes on secure websites.

Thank you sir it was antivirus issue i have disabled SSL/TLS protocol filtering and it's working fine.

With most programs, there is a way to set up Firefox to work with that feature if you like.