Recently discovered I couldn't access the website malwarebytes.org. After assuming the worst and running virus and spyware tests, but coming up empty, I turned towards my add-ons and the process of elimination revealed the problem to be with HTTPS Everywhere.
This is the error message I receive when HTTPS is enabled:
Unable to connect
Firefox can't establish a connection to the server at malwarebytes.org.
The site could be temporarily unavailable or too busy. Try again in a few moments. If you are unable to load any pages, check your computer's network connection. If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the Web.
In addition, I'd mention that it appears to block the site whether the rule is engaged or not.
Just a heads up.