When I choose a client certificate in Firefox (or cancel choosing none), Firefox seems to remember this decision even though I've told it to ask me every time. I understand that it'd be useless for Firefox to ask me every page load, so of course it's remembering my decision for the length of whatever it thinks of as a session, but how can I change that selection or close that session? It's not a per-tab decision...
Thanks!
You need to clear SSL session state of your browser. Take a look at my anwer to a similar question:
in Firefox choose History -> Clear Recent History... and then select "Active Logins" and click "Clear Now".