One of the things that I get asked the most is how to prevent a user from accessing the local file system from within Firefox. This generally means preventing file:// URLs from working, as well as removing the most common methods of opening files from the Firefox UI (the open file button, menuitem and shortcut). Because I consider this outside of the scope of the CCK2, I wrote an extension to do this and gave it out to anyone that asked. Unfortunately over time it started to have a serious case of feature creep.
Going forward, I've decided to go back to basics and just produce a simple local file blocking extension. The only features that it supports are whitelisting by directory and whitelisting by file extension. I've made that available here. There is a README that gives full information on how to use it.
For the other functionality that used to be a part of FileBlock, I'm going to produce a specific extension for each feature. They will probably be AboutBlock (for blocking specific about pages), ChromeBlock (for preventing the loading of chrome files directly into the browser) and SiteBlock (for doing simple whitelisting).
Hopefully this should cover the most common cases. Let me know if you think there is a case I missed.