Firefox 3.0.3 (on OS X at least) launched by Selenium RC 1.0 beta 1 displays an alert dialog box:

Alert
sb-ssl.google.com:443 uses an invalid security certificate.
The certificate is not trusted because the issuer certificate has expired.
(Error code: sec_error_expired_issuer_certificate)

I’ve seen a report that the nightly build doesn’t have this issue but, as a quick fix for Se 1.0b1, one can edit /Applications/Firefox 3.0.3.app/Contents/MacOS/defaults/pref/firefox.js
and change a couple of "browser.safebrowsing" preferences to false.

pref("browser.safebrowsing.enabled", false);
pref("browser.safebrowsing.malware.enabled", false);

That disables Google Safe Browsing – a feature which isn’t important to me but maybe it is to you, in which case this solution isn’t your cup of tea.

Related

Browser.safebrowsing.enabled
Google Safe Browsing for Firefox