Firefox version 29.0: User interface change
Firefox released version 29 at the end of April. It has a new User Interface which will probably mess up what ever customizations you have defined in version 28 or earlier.
However, there is a new Add-On called Classic Theme Restorer (which is only available for version 29 or later) that allows you to change the User Interface almost completely back to the version 28 look and feel.
There is another Add-On called Classic Toolbar Buttons which allows some great customizations.
With both of these Add-Ons, read the information on the Mozilla Add-On Website carefully to find all the tweaks to help make the new version comfortable to use. On my Firefox, I now have colored tabs on the bottom of the toolbar, some new icons that look real nice and a combined stop/reload button, just to name a few tweaks.
Mozilla claims some security improvements in 29 and it does seem a little snappier (subjective observation).