Rated 5 out of 5 stars
This plugin is the image of a beneficial and effective community driven service.
It keeps you out of unsafe websites very effectively: since I started using it I'm no longer afraid to unwittingly find myself in a virus / malware infested website - best chance someone else already reported it.
The fact that you can set the warning level and even prevent firefox from allowing entry to an unsafe website is also very welcome.
The only downside is that it's rather easy to report a website as unsafe, perhaps due to some other agenda, and then it will be falsely reported as unsafe until quite a lot of other users fix it.
But the plugin overall is great.