So, ran into this one. Firefox is throwing this error, along with ‘SSL_ERROR_RX_RECORD_TOO_LONG’. Turns out, apache was serving plain HTTP on port 443, as it hadn’t been given a default SSL config.
Other causes may be: Corrupted SSL cert (rare). Mis-configured proxy. Not adding “SSLEngine On” after configuring an SSL cert. But mostly, you’re trying to talk HTTPS to an HTTP serving webserver.
`a2ensite default-ssl` (on debian) fixed it. Well, fixed in in that the default server now has a snake-oil self-signed cert, but, you know, fixed it. 
Hello,
Thanks for the tip, saved a few hours.
Pete.
Glad it helped!