Add in jss-firefox-actor-state-transition and cleanup up jss-firefox-actor-start-listening, jss-firefox-actor-stop-listening.
Give ThreadActors a reference to the current tab.
Handle newScript message by creating a jss-firefox-script object
Initial (still buggy) implementation of jss-firefox-connection-disconnect
Move with-existing-io to jss-browser-api.el so the firefox code can use it as well.
Minor indentation/docstring cleanup