If Firefox has one "killer feature" this is it. It completely transforms the way a browser works, essentially creating sandboxed groups of pages that can only see others in the container and nothing else. It makes using the same site with multiple credentials painless and overall increases the security and privacy of browsing exponentially. If I had one gripe it's that the UI is a little clunky and it could really use some love. Otherwise, a huge thanks to the team behind this. You should be marketing this way more than you do.