I've got the opposite problem!
Way back in the dark ages, I had a PPRuNe ID whose password was in the cookie, and which I'd forgotten. The inevitable crash happened, and the cookie disappeared.
After a day of trying to remember the password, i gave up and created the new ID "Keef". That was about three years ago.
A couple of weeks ago, I had my "User" delete disaster, which took three days to get the PC back to normal.
Now, it remembers that old, distant ID too! Every time I restart Mozilla, I have to decide which ID to log on. Previously, the system didn't seem to log me out from one month to the next.