As many know, your non-default search providers show up by clicking a drop-down by the search box. At the bottom of this drop-down, you'll notice Manage Search Engines... link.
Click it. You'll see this.
Notice the keyword column? Notice the Edit Keyword... button? WTF?
This Mozilla KB article explains it.
See the keywords I added above.
Typing "w Iowa" searches Wikipedia for Iowa.
Typing "t health care" searches twitter for health care.