This page describes how to get Firefox 3.0.16 to permanently accept self signed certificates. Other versions of Firefox 3 should be similar.
For an explanation of why you would want to do this (including when you should not do it) and instructions for other browsers, please see the main Self Signed Certificates page.
Steps for Firefox:
Navigate to the web site in question.
You will see the Secure Connection Failed screen. At the bottom of it, you will see a link saying "Or you can add an exception ...". Click on it:
You will get two more buttons: Get me out of here! and Add Exception.... Click on Add Exception....
You will see the server address. Click on Get Certificate.
At this point, the Certificate Status should now be shown:
(Optional) Click View.... Click Close.
Ensure the Permanently store this exception checkbox is checked (on), then click on Confirm Security Exception.
You should now be taken to the secure site, and you should not see any more warnings for that site.