As you might know, Ubiquity is no longer in active development, so the standard version 0.5 is almost not working for latest versions of Firefox but finally I stumbled through this page that lists all versions of ubiquity so you can choose the one that matches your firefox version as in my case using Firefox 11 I used the latest version 0.6.2 pre that worked just great!!
https://addons.mozilla.org/en-US/firefox/addon/mozilla-labs-ubiquity/versions/
enjoy ubiquity!
~yooozy