Email is an integral piece of the open internet, and also a critical bit of infrastructure for the web, providing crucial communication and authentication for millions of websites.
Without free and open email, it makes every internet based company's job that much harder (potentially impossible).
I don't want to be overly cynical, but how is supporting a native email client helping the web? Email isn't "web" unless you access it through a web browser. So it seems totally in keeping with Mozilla's current web-only strategy to ditch their native email client and encourage everybody to use webmail. After all they want people to ditch all native apps (except the web browser) and use the web as their operating system. Supporting a native email client don't fit in with that strategy.