It gives you a huge network/user base who would never install a stand-alone client, but who let Facebook lock them in to their chat system because it's dead simple.
Remember, this is not about making life easier for geeks like us, people who actually know what XMPP is. It's about making it just as seamless to use Open web technologies/social networks for normal people as it is to use closed/private systems like Facebook.
Mozilla got bloated because the creators wanted users to live their entire computing lives inside a single application. The reasons for wanting that are always the same: it gives the creator the most power.
Of course this time around there is a slight difference; rather than adding a chat client to the browser they will prefer to keep adding features to the browser until somebody can write a chat client inside it. Either way the original criticism still stands, which is that doing everything in a single application means duplicating a lot of the work of the host OS, but generally not as well (from window management to scheduling threads to reclaiming memory to supporting hardware features like parallel computation to managing files).
The current plan at Mozilla seems to be for Firefox to be an OS that people use for everything, not just one of many applications that people use. This would give them a great deal of power. The power to kill services or applications that they don't like (e.g. Facebook), the power to prevent application developers supporting hardware they don't like (e.g kinect) and so on. More so than the traditional desktop OSs which have been open in allowing anybody to write software that talks directly to the hardware: browsers are sandboxes that prevent hardware access.
Note: none of this means I like Facebook. I refuse to use it. However I don't think Mozilla should be killing things on their platform just because they don't like them, especially when they are trying to set themselves up as an OS provider.
Firefox is (among other things) better than the rest of the browsers, because I can customize it.
I (having started this subthread) didn't suggest to start creating a Nescape Navigator kind of suite here. I'd just love to have a decent xmpp client as optional extension. Granted, this doesn't have to come from Mozilla.
The browser already is a chat client for millions of people. It's called Facebook. So since the browser already is the only chat client tons of people use, why not make it a client for a Free/Open chat system instead of Facebooks chat system, that they control, censor, give access to to NSA, etc.
Remember, this is not about making life easier for geeks like us, people who actually know what XMPP is. It's about making it just as seamless to use Open web technologies/social networks for normal people as it is to use closed/private systems like Facebook.