You have a misunderstanding of what is going on. Hello, for instance, is not some sort of project is draining resources and/or bloating firefox. It is essentially a WebRTC demonstration. Since WebRTC is a technology that needs to be implemented anyways, and is worth promoting, Hello becomes very interesting.

It is basically just a little bit of javascript that runs on top of all the WebRTC code that they'd have to build anyways.

Trying to do cross-platform screensharing, for instance, can be quite a hassle, but this will allow it to be done all without any sort of plugins, or any need for TeamViewer, GotoMeeting, etc.

And as for XUL and XPCOM, they've already been moving away from it internally for years. It is horrible. Talk about bloat and security problems. And they already are needing to rewrite their APIs for things like Electrolysis, which I assume we want. So why not actually do a good job with the rewrite?

The ultimate goal is to have the UI based on web standards (browser.html). This will reduce bloat and reduce the attack surface. And the UI will only get more customizable as a result.