Basically anyone who uses Firefox, or anyone who has uBlock, AdBlock, uMatrix, Purify, etc. the installed is invisible to JS-based analytics. They simply never load the GOogle Analytics, NewRelic, or whatever code as part of their “tracking protection”.

In financial services, we see about 15% higher unique users per day when analyzing HTTP logs versus Google Analytics. In fact, I’ve heard but not verified that GA now applies a “correction factor” to their stats to account for this.