Removing the master password fails (win10)

I set a master password many years ago, and I still remember it so have no problem opening the profile which it locks. However I am fed up with typing it, because it is long and complex. I tried to change the password, but Thunderbird says unable to change Master Password. I tried unticking the Use a Master Password box, it asks for the existing master password, but although the box disappears, the Use Master Password box remains ticked, and the profile asks for the Master password on startup. I guess I could create a new profile, and suppose that would not need a password. But what is the point of a tick box and a system for changing the Master Password if it doesn't work!