TelemetrySession is currently owning the code [1] to persist the session state to disk to a file that survives profile resets and migrations. Since we're starting to use this file for things other than the "session" (see bug 1120370), we should probably refactor this code and make it live outside of TelemetrySession. We should probably also rename the file from "session-state.json" to "telemetry-state.json". [1] - http://searchfox.org/mozilla-central/rev/7057a51c5cf29b5b115b1db19ace2cfe3fd83a0e/toolkit/components/telemetry/TelemetrySession.jsm#2115