A very simple idea for a browser extension

DOM-walking shortcut keys: hjkl. If you've used vi or Google Reader, you probably know where I'm going with this.

H: Switch focus and highlight to parent DOM node.
J: Switch focus and highlight to previous sibling DOM node.
K: Switch focus and highlight to next sibling DOM node.
L: Switch focus and highlight to first child DOM node.

Something like that would make browsing most blogs' and forums' comment sections much more convenient.

It's simple enough I'd probably do it if I knew how to make a Firefox extension. :-|