After the Firefox 4 update to beta 7, you may notice that the status bar by default has been discarded.
The concept of status bar has been moved to the address bar. Hence, once you hover your mouse over any links the preview of the links will appear in the address bar by default.
However, you can still view the icons used by the addons, which was previously seen on the status bar, by enabling Add-On Bar. Go to View->Toolbars-> and check Addon bar to see the same.
As of March 22 2011, the official release of Firefox 4, the status will appear Just above the Addon bar