Baseline requirements:
- Open source
- Runs on Linux (Windows and OS X would be nice too, but I don't fucking care that much)
- Extensible (Even "has no plugin system, but really clean source and a
maintainer who is very open to PRs" is good enough for me)
- WebExtensions do not fucking count. They are only interesting to me if the browser implements a far wider and more powerful set of WebExtension APIs than is typical -- and even then, I don't like JS