Sorry, I don't think Pocket and Hello surpasses the junk bundled with Communicator, which came with a custom mail client. Mozilla's mail client is not only not installed with Firefox, but it's also pretty much dead now, in fact.
Netscape had a chat thing, html publisher, and I think something else.
WebRTC, pocket, the webIDE, even things like the dom inspector, style editor etc and pdf are bloat for most mortals. Perfect for some "official" plugins. They managed it for the FF OS simulator.
It's all optional crap that increases bloat and attack surface, except it's not optional.
"pdf are bloat for most mortals"
That's alright, it means in 99% cases it's just a very small javascript file on your disk. No harm done.
The sad fact is that desktop mail clients are going away, and even the big projects like Thunderbird are barely limping along, which I guess is slightly nicer terminology than "pretty much dead"?
[0] https://www.mozilla.org/en-US/thunderbird/releases/
[1] https://blog.lizardwrangler.com/2012/07/06/thunderbird-stabi...
[2] http://techcrunch.com/2015/11/30/thunderbird-flies-away-from...
And there's about 10 million users that evidently have a reason to keep using it:
https://blog.mozilla.org/thunderbird/2015/12/thunderbird-act...
So I completely agree. I cringe every time I am forced to use webmail for some reason.
Just an example: try to create (and use) an IDN-based email address with non-ASCII characters in the local part. You will be amazed how incompatible on so many layers this actually is. I can hardly imagine there weren't dozens of similarly tricky situations where the developers of this, or any other email client for that matter, were forced to make quite serious design choices because of either ambiguity in standards, or widespread lack of support thereof.