Wizz RSS for Firefox is no longer maintained or supported. However, a new Wizz RSS project has recently been started: -

Wizz RSS for Ubuntu.

Downloads of Wizz RSS for Ubuntu are available here - https://launchpad.net/wizzrss

If you find the whole launchpad thing as confusing as I do, you can the latest .deb installation file here: - http://wizzrss.com/wizzrss_12.07.11_all.deb