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They're cutting down on unnecessary chrome. I can't say I have a problem with it; less chrome means more room for page content. A bit like why Apple added fullscreen apps.
I run Firefox with a lot of plugins. I rely on the status bar (add-on bar) to keep many of the plugin notification / configuration buttons out of my toolbar. From my perspective, Firefox is working hard to drive me away, by removing its distinguishing features that make it different from Chrome.
I can't say I agree with all of their choices, but I'm running 29 right now and I don't feel like any functionality or customizability is lost from the last version. If you're not looking for the defaults you can change pretty much everything you want with a few add-ons and about 10 minutes or so of configuration.