The Status Bar at the bottom of the Firefox window has changed since Firefox 4 and some of its features have been moved to other places. This article explains where to find everything and links to an add-on that recreates the old Status Bar.
The new Firefox Status Bar only shows page loading activity and link previews. When neither of those are needed it simply disappears.
When you mouse over a link, a preview of the URL popups at the bottom left side of the page.
When Firefox is loading images and other assets from a website, the status popup appears at the bottom left side of the page and displays the URLs where these things are coming from. If the status popup is in your way, you can move your mouse over it and it will move to the right side of the page. When the page is done loading, the status popup disappears.
The status of downloads can be found in the Downloads Window which conveniently opens when you download a file.
The padlock wasn't guaranteeing that you were connected to the right server. Security information about a web page is now only shown by the Site Identity Button.
Many Add-ons used to place an icon in the Status Bar that gave you quick access to their features. Now they place their buttons in the main toolbar. To learn more, see What happened to the Add-on Bar?
The old Status Bar is dead, long live Status-4-Evar! Status-4-Evar is an Add-on that recreates all of the features of the old Status Bar and lets you put them in the new Add-on Bar.