Impero is a fork of the Zotero citation software project, with changes which allow a user to host their own sync server, rather than relying on the people who run the Zotero project, and also improvements in the citation process.
To read blog posts about the ongoing development, click here.
A pre-built .xpi file is hosted here.
Development is also underway on the sync server, to simplify its installation.
No support is offered for the extension or the data server from Zotero, and for the time being it should be considered alpha status, not suitable for use.