Saving and retrieving text typed into a textbox in a Firefox addon

I'm just now getting used to using XUL. I've created a preferences window (opti开发者_如何学运维ons.xul) with just a text box:

 <textbox id="api" preference="link-target-finder-api"/>

But I'd like to save the text entered here and access it. How do I do this? Thanks!

Use a XUL textbox:

<textbox id="hello-world" value="Hello, world!"/>