Shift + Delete
Simple right ?
You can also do this more than once. For example, while typing "www.c" on Firefox's address text input, it will display other URI starting with those characters. Here, you can use the arrow key to highlight the URL you want to remove and press "Shift + Delete". Repeat as necessary for other unwanted URI.
It was unusually difficult to locate this feature by navigating Firefox's menus, so I Googled. The result is posted here just to spread the word.
