I installed 380.70 on my RT-AC68P (running as a Media Bridge) nearly 5 days ago, today something odd happened for the first time: I booted up my desktop, fired up my Firefox and went to my router page and it let me in without the usual admin login/password. I closed and restarted my browser and again could get in without login/password. Prior to today, the session would never persist following a browser close, and would automatic timeout after prolonged inactivity.
Interestingly, this behavior only happens with Firefox (version 65.0.1 (64-bit) dated 2/12/19); with Chrome or IE the behavior is still the same as it has been before today. This seems to point to new Firefox as the culprit of this behavior change.