Connection blocked security notice

Hello! This is my 1st time here and I am exhausted with this pop-up:

"firefox.exe attempted to establish a connection relying on an unmatching security certificate to We blocked the connection to keep your data safe since the used certificate was issued for a different web address than the targeted one."

It pops up no matter where I am trying to go. How do I stop it?

Thanks! GrannyChris