Separated release from pass-browser-chrome

Newer firefox versions have better and better support for web-extension based add-ons, so I decided it was time for the firefox add-on to have its very own git repository and update it with 0.6.0 at the same time!

This repository will be merged every once in a while with upstream and have the same tags/version numbering (patch version might differ because of a different review process to get a new release in the add-on store).