router99 Hardening a browser (in this case firefox) means going in and tightening security settings to make it more "secure/private" (hardened). E.g., enabling settings like https only, clearing all browser data on exit (cookies, history, etc), de-selecting all forms of opt in telemetry like anonymous data anaylitics send off, installing some extensions like uBlock, etc. Bassically, tighten your security settings to what you deem is necessary for your purposes.