Getting the Source
To get the source code for Ubiquity and develop for it, you'll first have to install Mercurial and Python if they're not already on your system. Then, from the command line, enter:
hg clone http://hg.toolness.com/ubiquity-firefox
Now enter the newly-created ubiquity-firefox directory and read the README file contained in it for further instructions (an online version can be found here).