Chrome has done this and thrived. I still use Firefox for the extensions and the fact that Google can not be trusted.
Complexity isn't easy, but it's not that black and white. Just because the first time round it didn't work out so well doesn't mean it won't this time; especially since the circumstances are so very different.
In any case, things like personas are supported, but most of the complexity isn't in the browser; new functionality doesn't need to be tightly coupled to the browser - just exposed.
I actually think the problem isn't so much on the software side, it's organizational: trying to do too many things at once means making sacrifices.
First one: That only needs to switch the Feed button to a "Like" button that subscribes you to the feed in one click. Then it needs to change the "new tab" page from a speed dial to a stream of those feeds. This stream could be hosted in the cloud (probably it should, to be available anywhere), and so would not add any bloat.
The chat I understand can be seen as bloat. But still, it's just one function, not those hoards of functions that the original Mozilla suite had.
And the point here is that it's really important to make people less dependent on Facebook, to pave the way for other initiatives and efforts to create another chip in the Facebook wall, and then some others another chip, etc. until the wall finally comes crashing down. IMO that's so important that one extra function, adding a little bloat, is well worth it.