I’m a big fan of having my applications run in fullscreen mode, especially firefox. But I always hated how the tab-bar is always in auto-hide mode. And after Googling a little I found out how to disable the auto-hide feature! It’s super simple. All you have to do is:

Type about:config on Firefox address bar.
Search for browser.fullscreen.autohide
Set it to false

Source: superuser.com