How to import SSL certificates for Firefox with Selenium [in Python]?
Webdriver has no built in mechanism for adding a personal cert.
If you are using firefox the only way that I have found to do this is to create a firefox profile and add the certificate to it. You can then either reuse the profile when you run your tests OR, and this is my prefered option, take the cert8.db and key3.db files and add them to the profile that webdriver creates at runtime. - Derek Ekins
although the next answer provides a workaround.