I've been using this for the past couple of weeks (initially on Chrome and now on Firefox) and it is simply the single best way out there to quickly find and read legally open versions of paywalled journal articles. The interface is super easy and intuitive and it's now a part of my everyday toolkit.

COI: I volunteer on the Board of Directors for Impactstory, the non-profit that makes this add-on.

Unpaywall does now indeed work on both Firefox and Chrome. Kudos to you.

Bonus points for "for the Open Access nerds: the Unpaywall tab can change colors to indicate Green or Gold OA, too. Enable it in settings."

This add-on has helped me find a lot of research papers I needed for my thesis. Please keep up the good work. I am amazed it has been this useful in so little time but I found a solid 90% of the research papers I needed!