Establish secure connection to localhost in Firefox

I have a Greenbone Security Assistant that has me connect to port 9392, with the command:

sudo openvas-start firefox http://localhost:9392 

in Firefox. But before (and sometimes after) connecting, Firefox throws a lot of errors about insecure connection, and always highlights the better part of the URL in red. This also happens when connecting to localhost for, say, Autopsy. Is there any way I can establish a secure connection to localhost? Maybe from the terminal, in the firefox http://whateverURLforyourapp command?