Add support for disabling client cert prompt in Firefox

[drGrove]

Firefox supports disabling the client cert prompt via the following preference in prefs.js of a profile:

userpref("security.default_personal_cert", "Select Automatically");

This option should be user configurable and also contain a warning. This was set to Ask Every Time in FF 2.x because it was considered harmful since it's all encompassing, unlike Chromes implementation that allows this on a case by case basis.