It does do what it's intended to do. However, it isn't without problems.

The UI is rather clumsy, making you enter blocking criteria manually (recent update) and the default when you fire up the UI is "search" instead of "add." Seeing as you're more likely to add block criteria than search for them, this is strange.

It also has trouble blocking foreign characters and symbol characters. Even when you copy and paste, the blocking is hit and miss on a channel with those characters.

Since the update, it's been a huge resource hog, slowing down firefox quite frequently, and sometimes locking it up altogether. It shows a major security flaw in either the plug in or Firefox itself as a plug in should not have this much effect on Firefox.