Our Web App uses SSL and is deployed daily to a development server that has a certificate signed by our internal Certification Authority. This causes some problems with Selenium, as this is essentially an "untrusted" certificate in the eyes of Firefox.
It is easy to get around this, you just answer the prompt that Firefox throws up and tell Firefox to trust this certificate. But this isn't good enough for an automated build, as Selenium can't answer this prompt for you every time.
So to get around this I am running my Selenium tests against a custom Firefox Profile that I have created and checked into subversion. There are instructions how to do this here. Inside this profile I have told Firefox to trust the certificate, and remember that to "permanently store this exception".
I found that the "Permanently store this exception" box was greyed out when I first tried this. This was as a result of previously configuring firefox not to save any history. The solution for this problem is this:
1. Enable History
2. Permanently store the exception (The box is not greyed out now)
3. Disable History
I only needed to do this once - Firefox seems to remember the certificate exception even though I set it to not remember History afterwards.