The browser's options may clear its cache and cookies when you close it down. That's one possibility. If your IP address changes the next time you access TT, the system may treat you as someone new, hence asking for PW. I do not think MAC addresses change since that is HW. Anyway, try changing the options associated with your browser's cache/cookie management.
My FF browser will clear everything when I shut it down since it needs to clear out stuff from secured sites so I never check the 'Keep Logged in' box anywhere. If you haven't done so, have the browser keep track of your ID and PW and store those under a master PW (long non dictionary related password like a phrase w/ mixed characters). HTH