I am a long time user of Firefox, and have been very happy with this browser, mostly due to its flexibility and library of add-ons. Though I haven't liked the latest developments in the user interface, I've still been able to set up the program in a custom configuration that suited my needs. That may be ending soon however.
The Australis edition of Firefox is coming, probably in Firefox 29. It is already available in the Firefox nightly editions, and will be added to the mainstream version in the coming months, perhaps as early as the end of April. Australis brings a new look, similar to Chrome in many ways, with an intentionally minimalist design. But this new interface does set new limits in the ways the browser can be configured.
For those who like tabs on the bottom, Australis removes this option entirely. It is not going to be available, even through about:config. There is quite a lot more, far too much to repeat here, so take a look at the direction they are taking: http://blog.mozilla.org/ux/2013/11/australis-is-landing-in-firefox-nightly/
In response to this, a new add-on has been constructed, that may become far more popular than the Mozilla developers would like to see. It is called Classic Theme Restorer, and will undo most of the changes seen in Australis. http://addons.mozilla.org/en-US/firefox/addon/classicthemerestorer/
I have taken a different approach and am now trying out a fork of Firefox called Pale Moon. This may be old news to many of you, but I find it to be quite good so far, and the migration tool to import settings from my installation of Firefox worked perfectly. Pale Moon uses current bug fixes and performance improvements from the main line of development of Firefox, but will not use the Australis user interface.
Pale Moon can use the same add-on's that Firefox does if that is a concern. I haven't been using the browser long, but so far, I have been very pleased with its performance.