You have a software FiddlerRoot, or BrowserProtect installed on your computer. It does intercept you

You have a software FiddlerRoot, or BrowserProtect installed on your computer. It does intercept your secure connections and sends it's own certificate instead of the website's certificate. This is why Firefox is complaining about it.

Getting rid of FiddlerRoot should fix it.

Also see https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware