The Addon Bar (Restored) will give you a place to put all your add-on buttons without over-crowding the navigation bar. It aims to be a lightweight, yet fully functional replacement for the old add-on bar.
Here are more details about this extension:
Great for power users who have many addons installed, and especially for long toolbar items.
This also restores the status-bar area, so it works with extensions such as Status-bar Scientific Calculator, Version in Add-on bar, and Forecastfox Weather. Plus, it adds a new feature that lets you move the status-bar around!
The great thing about Firefox is that somebody almost always writes an add-on when a feature goes away to bring it back. you can pick up the Addon Bar (Restored) at the Firefox Add-ons website.