I am asking the security list about this, I will be back with a response when I hear from them :-)

There is a choice on the certificates tab on the Advanced options panel (before you click the "View Certificates" button) that says "When a server requests my personal certificate" with two radio buttons. The default choice is "Ask every time" but I'm guessing you have "Select one automatically".

Don't use that choice. "select one automatically" may or may not guess your preferred certificate, but even when it does it's a potential horrible privacy hole because every SSL site on the internet can now silently ask for your certificate and read any information in it.

If the site you are using repeatedly asks for your certificate (other than the first visit each day or first visit after restarting Firefox -- it's a form of log-in) then the site is broken and needs to fix its "SSL Session Cache".