Firefox 29 will introduce changes to the web browser that will rock the world of some users of the browser. The version of Firefox ships with Australis, a design, layout and feature change that introduces major changes to Firefox and is in my opinion comparable to the major switch from Firefox 3 to 4.

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Good news is that you can modify the browser to your liking or restore the interface so that it looks similar to the one you used to work with. Bad news is that you have to rely on third-party authors to do so.

Full article here:
How to turn the new Firefox 29 into the old Firefox (gHacks Technology News)
Related:
How to restore Firefox’s classic theme after the Australis interface update (gHacks Technology News)