The nice thing is that Mozilla makes this data easily available. You can see from the "Latest Version" graph on https://data.firefox.com/dashboard/user-activity

The nice thing is that Mozilla makes this data easily available. You can see from the "Latest Version" graph on https://data.firefox.com/dashboard/user-activity https://data.firefox.com/dashboard/user-activity that a) about 75% of Firefox users are getting updates to the latest version and b) that number did not appear to change with the release of Firefox 57. So there are approximately zero users staying on pre-Quantum versions of Firefox. (Perhaps there are users of Pale Moon or other pre-Quantum forks?)