Firefox comes with with 3 keyboard shortcuts which will help you auto-complete the .com, .org and .net addresses in the location bar without the need of typing them. These are:
Ctrl+Enter - auto-complete .com address
Ctrl+Shift+Enter - auto-complete .org address
Shift+Enter - auto-complete .net address
For example, type google in the address bar, then Ctrl+Enter. Both www. and .com parts will be auto-completed.