Why do all the popular browsers blatantly copy each other's user interface?

See these screenshots to understand what I mean:

Mozilla Firefox (see Australis is landing in Firefox Nightly and Measuring Australis)

Update: Australis has now been pushed into mainstream Firefox. See https://blog.mozilla.org/ux/2014/03/introducing-the-update-experience-for-australis/.

Google Chrome

Microsoft Internet Explorer

Is there some reason why don't they innovate and create their own unique interfaces?

I'm not saying that they should "go against the flow", or anything like that, but why do they have to all look so similar? Why not rearrange the interface elements? Why do they all need an "App" button at the far right of the toolbar?

I'm not asking for an answer to all those (separate?) questions, but I don't understand why they all need to be (almost) exactly the same.