Sure, you might spend time on instituting complicated certificates and warnings. Of this you are of coure entirely correct. Except it's a lot harder to do right for many-to-many e-mail correspondences, that go through a multitude of different recipients and servers – some of which don’t use encryption at all – compared to the one-to-one browser communication, that goes directly to the end recipient, and stops there.
At that point the notion of “one-to-one encryption for e-mail” becomes completely meaningless, because it’s almost never actually one-to-one. The e-mail is frighteningly often sent to some intermediary party, inside or outside your organization, who you do not have any control over, and who will then forward that e-mail – encrypted or not – to the actual end recipient. What that means, is that no matter the amount of encryption schemes you throw onto an e-mail, the likelihood of a leak is still very high. And while undetermined or opportunistic third parties might not gain access to such a correspondence, a determined organization or state actor surely will.
As such, more encryption does not inherently make e-mail more secure. Especially when you know that there are multiple parties handling any given correspondence, with multiple clients and multiple servers – some of which do not use any form of encryption at all. You can never be sure that some user might simply forward the e-mail unencrypted – for whatever reason – down the chain of correspondence. For that reason, e-mail is inherently insecure and non-confidential, and it should be treated as such even if you strongly encrypt your own messages.
In fact, encrypting e-mail might lull you into a false sense of security, because the second that e-mail leaves your client, then the fate of that message is also completely outside of your control.
In order to fix this problem, you essentially need to create an entirely new e-mail system world-wide, with an entirely new protocol. Mozilla simply does not have that capability on its own, though motions to increase e-mail security is of course laudable.