Hi this help article should be what you need.

• Secure connection failed and Firefox did not connect
  • or How to troubleshoot security error codes on secure websites

Please post back to say how you get on.

If you need further help please say the exact message you receive and which sites it is on. (Is it all secure sites including this forum.) Additionally please list all the security related software you are using.

I don't understand anything on that link.

jckinnick said

I don't understand anything on that link.

Sorry that link did not help. Are you able to provide more details please ?

John99 said

If you need further help please say the exact message you receive and which sites it is on. (Is it all secure sites including this forum.) Additionally please list all the security related software you are using.