I have a feeling that if Firefox can't slim down the browser, stabilize it, and FINALLY fix the horrific memory hogging, than those will be the death knell of the browser. I honestly think they need to pull an IE, and trash support for both XP and Vista, and strip that old code out.
Maybe it's just my particular selection of addons or something, but I haven't had any particular stability or memory issues with Firefox since like version 4 or so. Only crashes I've gotten in the past year or so are out of Flash and not the browser itself, and memory usage is much lower than Chrome, especially when multiple tabs are going.. the only better one resource wise (for me) is IE but chalking that up due to it having a lot less addons installed. I wouldn't be surprised to see some of the legacy baggage get tossed eventually though once XP goes past its expiration date.. it's still supported by Microsoft, so it should still be supported by third parties too. After that date though, anything goes.