Just updated to Firefox 29? Not a huge fan of Australis? No problem, there's a
classic theme restorer addon that reverts (most) of the old feel back. I've successfully used this under Windows, and it's quite tweakable. (
Arch hasn't yet updated Firefox, so I haven't tested it under Linux. It was just updated yesterday, but I have to build it from the AUR as I use KDE enhancements... testing will be forthcoming!)
I do have to admit, I don't particularly mind the changes to the UI (aside from having tabs above the address bar like Chrome) and it looks clean. Some people despise it. This addon gives you options.