I think a significant change had been made in the last version of Firefox. Having same problem here. I solved it in settings/privacy and security/certificates/see certificates. There is a new label, see image, where is registered a pair of site and certificate chosen last time for that site if you marked "remember my decission" box in that moment. In this case, Firefox doesn't ask you for choosing a certificate when you access this site again. So, remove the site and next time you enter this site Firefox will ask you for choose desired certificate. NOTE: the image is from spanish version of Firefox, in english version label could be named "Autentication Decissions" or "Signing decission" or something like that. Hope this helps you. Bye