It's not very polished. The only reason I registered it as an official add-on is so I can use it without having to load it manually every time I start FF. However, if anyone else is interested I'll polish it up, publish the source and other such niceties.
In general, it can be used to apply CSS to elements whose attribute values or text match the regular expressions.
To configure it: click the "Preferences" button in the Firefox "Add-Ons Manager"; copy the example JSON into the textarea; modify it any way you'd like; and click "Save". The example sets visibility to hidden on links to the sites in the list. If you hit up HN, you'll probably see a few hidden links on the front page.