Just so others can find this trick more easily: In order to make accepting self signed certificates a little easier in Firefox 3.0, visit the page about:config and set the following values:

• browser.xul.error_pages.expert_bad_cert  true
• browser.ssl_override_behavior 2

Here are the relevant links to the mozilla knowledge base: http://kb.mozillazine.org/Browser.ssl_override_behavior http://kb.mozillazine.org/Browser.xul.error_pages.expert_bad_cert

As soon as these values are set, firefox enables the Add Exception button (you had to click a blue link before the changes) and pre-fetches the certificate which saves you another click in the exception dialogue.