"SSL" (Secure Sockets Layer) is a standard for establishing an encrypted link between a web server and a browser to ensure that all data passed between the web server and the browser remains private.

The "geeks at Pingdom" describe a problem with the way Firefox version 3 handles "SSL certificates" (which the casual user does not even see under normal conditions):

• New SSL policy in Firefox hurting tens of thousands of sites, Pingdom, August 19, 2008.

If you visit a website with either an expired or a self-signed SSL certificate, Firefox 3 will not show that page at all. Instead it will display an error message, similar to any other browser error (for example a “page not found” 404 message).

...[T]his is not something that only affects smaller websites. For example, the SSL certificate for the official US Army website [https://www.us.army.mil/] is declared invalid by Firefox 3.

See also:
What is SSL? (ssl.com)
SSL (Webopedia)
SSL (Wikipedia)