This addon had a lot more 1-star reviews than it currently does. They have presumably been removed at the request of NewsGuard.

The reasons why I would urge people to avoid this add-on:

1. Serious privacy problem: sends the sites you visit to NewsGuard's servers - there's no reason why this addon cannot work without such reporting.

2. The people involved with this addon (see their advisory board and read the reporting that's been done on them) are, I'd argue, not the people who should be entrusted with deciding what is/isn't a reliable source of news.

3. A site which refuses to participate in this scheme will get flagged as unreliable (see what happened to Boing Boing when they were approached)

4. Mozilla/Firefox co-founder Brendan Eich has commented on this company: "This is a bad operation all around"