This would be a lovely feature, but sadly Mozilla has taken this absurd position on it:
[Leaf nodes when using delegated credentials are] designed to change frequently, even in the absence of a known attack, so that signal is unlikely to be useful. If you feel strongly about it (there may be other reasons to include this information in the getSecurityInfo api), please file a new bug
This would be a lovely feature, but sadly Mozilla has taken this absurd position on it: