One of the changes of Internet Explorer 9 was Microsoft's decision to move the tabs of the browser into the titlebar. The small change freed up space for the websites and services opened in the browser. While not everyone may like the move, considering that navigational controls, the browser's address bar and tabs shared a single toolbar now, some liked the new minimalism.
Firefox, being the most customizable web browser of them all, can be configured in a similar fashion. You probably now that you can display tabs on top or bottom in the web browser by default, and also modify many of the tab related settings in the browser's advanced configuration or with the help of browser extensions.
Mozilla contemplated moving the tabs to the titlebar back in 2010 and implemented the feature when Firefox runs in maximized state. That does not mean that you cannot move the tabs to the titlebar when it is not though, as you only need to install an extension to do so.
Tabs in the titelbar
Tabs always in Titlebar is a Firefox add-on that does just that. Once you have installed the extension in the browser you will notice that the browser's tab bar and titlebar has been merged into a single toolbar.
You may notice a couple of other changes as part of the transition. It may for instance be difficult to move the browser window around, as you have less space that you can click on to do so. The best location to move the bar is just underneath the Firefox button.
The extension ships with a single option. It lets you change the distance of the tab bar from the top of the browser window. It is set to 8 by default. Decrease it to move it even further to the top and save more space, or increase it to improve accessibility of the top window bar for moving it around on the screen.
Attention: Installing the extension and enabling tabs on top will remove the option from Firefox's context menu. It is still possible to get the feature back by setting the browser.tabs.onTop parameter in about:config to false. It won't add the context menu item, but will move the tabs to the bottom automatically.
If you want to save additional vertical space in Firefox, install the tabs always in titlebar extension to do so. It may take some time to get used to the display there though, but since it is easy to restore the old tab display format, it should not really stop you from trying it out in the first place.