Add-on's settings get lost if during a session one opens the add-on preferences in a tab and then -either with this tab still still open or closed- clears cache-etc. data.
If ones clears cache-etc. data without having opened this add-on's preferences during the present session, its settings are sometimes retained, sometimes not (can't pinpoint the exact triggers/conditions, if there are specific ones).
Hence the 2 stars; it's almost useless if almost every time I have to reenter the reference configuration for sites.
When this gets fixed and if there are indeed no longer as others claim calls to google analytics etc, then imo this would be a fine alternative to the old pre-Quantum refererence control add-on, worthy of 5 stars.
Also the bubble-like url field to enter a referer is absent or "minimized" when one first sets a setting for a site. One has to close the add-on preferences tab and reopen it for the referer field to appear and function normally, i.e. in order for one to be able to fill in the url.
PS Firefox 59.0.2 (64-bit) on Ubuntu 16.04 LTS.