Firefox is the name of the web browsers for desktop, and mobile versions for Android and iOS. The company Mozilla had a email client which was Thunderbird before it became more of a community project. Mozilla used to have the Mozilla Application Suite but disconnected it many years ago and it continued as the community effort SeaMonkey since which includes the email client as part of the suite, though they have been struggling a bit to keep up compared to Firefox and Thunderbird. So are you asking for Mozilla to have a email client built into the web browser Firefox or for to make a separate email client again?. Having a email client as part of Firefox would greatly bloat it and make it harder to maintain it.