TwitterSearch searches search.twitter.com for Tweets matching your words. (see blog entry, Twitter Search Command for Ubiquity, for more information and source code)
If you have Ubiquity installed in Firefox the subscribe bar should automagically appear at the top of this page. Upon subscribing, you will need to restart Firefox before you can use the twittersearch command.
As always, your mileage may vary. Subscribe at your own risk. The usual suspects.
For more information on Ubiquity for Firefox visit Mozilla Labs