With Firefox 4, Mozilla has introduced Add-on Bar, new menu and ditched Status bar. While Add-on Bar that appears at bottom of Firefox window if enabled by you is for the placement of Add-on icons. It has been removed in Australis Interface -now available in Nightly for testing.
Enabling Add-on Bar in Firefox Australis UI
However you can still bring it back, for that you just need to install this add-on – which brings Tabs on bottom feature, old theme and Firefox Menu as well- and as usual after installing the extension you can enable Add-on Bar it by right clicking and selecting ‘Add-on bar’ on Navigational toolbar or by using ‘Ctrl + /’ Shortcut.