Mozilla Firefox 4.0 has introduced new features that prevent the Vision Firefox plug-in from loading correctly.  This manifests as the web filter functionality of Vision being unavailable for Firefox.  What occurs instead of you getting a site list is that you will see the full blocking screen as if you blocked the web entirely.

A fix for this issue will deployed in a future version of Vision. Currently the workaround we have is to recommend you download an older version of Firefox and use that until the new version is released.