Not everyone likes tabs on top. Some users prefer tabs to be placed below the address bar. There are a few possible reasons for this. For example, because the tabs are used more often than the address bar. Thus, why not putting something that you use a lot more closer to your reach? Moreover, the address bar can easily be accessed with a shortcut key. There are indeed shortcut keys to open the first up to the ninth tab mouselessly but it becomes much less useful when you open tons of tabs.
Disable Tabs on Top is a restartless Firefox add-on that moves Firefox tab bar below the address bar. The extension automatically moves the bar after it is installed. While it works nice, there is a problem that some users might want to solve. After the bar is moved, its original place on top becomes a wasted space. It is kind of weird to have an empty space that wide.
To handle the wasted space left behind by the tab bar migration, one of the possible suggestions is to use Hide Caption Titlebar Plus. This add-on automatically removes the empty space by aligning the Firefox menu button and the [minimize, maximize, close] buttons with the navigation bar.
Version reviewed: Disable Tabs on Top 1.0.rev4 on Firefox 21 [download from here]
With the combination of these two add-ons, you can have the tabs where you want them to be, that is below the address bar, and have a more minimalist interface. The Hide Caption Titlebar Plus has more options to make the browser even more minimalist.