and I'm sure the extension doesn't just send the URL of every page you visit somewhere...

EDIT: Oh, it appears to be hashed... that's not as bad as I expected - but it's still extremely abusable. Nope... just nope - absolutely not without some sort of differential privacy or other concealment, thanks.

Comments under example.com already give a good idea what it will look like when everyone will start posting comments in public places.