That depends. Are you talking the Firefox internal, or an add-on? That is it you want to do?

I'm looking for the one that comes installed with Firefox

Go to the test box, and click it. Now press a letter. If you entered something in the box starting with that letter, you will see a list of those entries. You can then select one of those with the mouse or arrow keys. Or, you can select one of those, and remove it be pressing Delete while it is highlighted.

If you want, you can turn this off.

You can also set Firefox to clear the list when it shuts down.