To make Firefox browser more compact and increase its usability, Mozilla lab has released another experimental add-on called OneLiner, which turns Firefox interface into IE9. You may remember the LessChrome HD add-on, which gives more browsing space by hiding all things except the tab bar. OneLiner add-on takes a different approach to give more browsing space.
As is IE9, this restartless add-on combines the navigation and tab toolbars into one line to give more space to web pages. To combine the navigation and tabs toolbars, the location bar is given a small fixed size while the search bar is collapsed to a single button.
1. Download and install OneLiner add-on. It is a restartless add-on, so your Firefox does not need restart. After installing it, the add-on combines navigation bar and tab bar into one-line.
2. To perform searches with Search button, enter keywords and click search button or simply press enter button.
3. You can use Ctrl+ K shortcut instead to activate in-tab search bar.
Note: Location bar expands to cover back or forward buttons when they are not in use. One-line will visible even in full screen mode, because it does not get auto-hidden. (via)
You can install Oneliner Firefox add-on from Mozilla add-on site.