It seems to me that the default button is live, but that the mouse cursor doesn't jump to it; however, a keyboard "enter" triggers the preferred/default response. I wonder if there is a setting somewhere to toggle a default between keyboard and mouse.

Thanks StephenB. Yes, it is now obvious that the default button has been selected without the cursor jumping to it, and as you say, hitting "Enter" does the job. Still not as natural a process as just clicking the cursor key and as your hand is already on the mouse it is probable faster to still move the cursor to the button. Maybe Mozilla will fix it in the future so that the cursor jumps once more. Regards