Update: I heard that the development of Ubiquity was stopped.
If you are a Mac user and you love Quicksilver, you will probably love Ubiquity for Firefox. This Firefox add-on is simply amazing. Just watch the following introduction video, install Ubiquity, read the tutorial, and use it. You will love it. Current Quicksilver users can learn how to use Ubiquity in no time because it’s very similar.