Add support for disabling client cert prompt in Firefox

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