There's so much that can go wrong with that, especially if this stuff eventually shows up in some data leak.
Here's some examples:
1. URLs can contain sensitive data, which will tie your session to your person (facebook URLs etc)
2. This could create vulnerabilities in (for example) checkout flows that pass data via URLs
3. I'd rather not make the job of intelligence services easier than it already is
4. Sometimes I clear my browser history for a reason
Trying to find matches for URL hashes would be a much nicer approach, but you would need a database of hashed URLs for that, so it's not a trivial fix.
Look up the protocol that Google's SafeBrowsing[0] project uses. The short version is that your extension should check visited URLs against a locally-downloaded database of HN-posted URLs. For SafeBrowsing there's a bit more optimisation (e.g. hashing URLs & storing only hash prefixes to reduce the size of the database), but that's the short version.
This secures the end user's privacy.