This is happening on a new HP G72 Laptop.
It a pop up alert with the option "try again" I see in the error console that there are several entries like this:
Warning: Unknown property '-moz-transition'. Declaration dropped.
Source File: https://support.mozilla.com/media/css/common-min.css?build=3fabbc0
Please try http://support.mozilla.com/en-US/kb/error%20loading%20web%20sites#w_interrupted-or-reset-connection
That didn't work,