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The recent market share of the various browsers indicates that your concerns are shared by many users. Firefox's share of the browser market has been slipping while Chrome, whose rapid update model Mozilla tried to replicate, has been increasing. One problem is that while Chrome updates are mostly invisible, Firefox is constantly telling you and/or breaking plug-ins. Also Chrome updates don't make Chrome crash prone.
I think instability is the biggest issue. I used to think of Firefox as the stable browser, but not anymore. A few years ago I never would have thought I would say this, but when I'm on someone's desktop and they have IE and Firefox, I'll often go with IE just because it doesn't crash as often as Firefox. Until just now I didn't realize it, but Firefox may actually be my #3 browser choice, although other than stability issues (which is a big deal), I prefer it to IE. Three years ago Firefox would have been my #1 choice.
I love open source projects, and I want Firefox/Mozilla to succeed. For many years I was a devoted user. But if the stability, and to a lesser degree, speed issues that I have seen recently don't get ironed out Firefox is going to lose more and more market share.