Thanks, Jared.
Today, for the first time, it came out of FIRM MODE. It did it only for a short while. It was when I came to a stop with the brake pedal depressed.
After I let off the brake pedal, it went back to FIRM MODE and has not changed since.
I inspected the brakes and found that one of the rear pads has loose rivets. Not sure if this is the problem, but I will be replacing them today.
Update to follow.
Theodore