Command+Return and Shift+Return keyboard shortcuts stopped working

I'm using Firefox 4.0 on a 21.5" iMac running Mac OS X version 10.6.7. Just this morning, the command+return and shift+return keyboard shortcuts to auto-complete the .com and .net addresses of websites stopped working. Command+w still closes a tab, and command+left click still opens a tab in a new window. However, when I type anything in the address bar ("mac" for example) and then hit command+enter, Firefox opens up a new tab with a Google search for the word I typed. The address for this new tab is as follows: http://www.google.com/search?q=mac&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a Any idea what gives? I'm rather perplexed and frustrated.