The method you are referring to is used in Africa by several different tribes. They build a house with cob then build a fire inside the structure ( I mean the entire inside ) once the fire has burned out, they let the building cool then remove the
ash, put on the roof and voila, house that won't melt in monsoon rains.
It seems to me, that since Ireland and England have cob houses with plastered walls which have stood for more than 200 years, why would you want to bother with the fire marshal issues that are sure to come up by using the fired cob method?
If you fire an adobe block, it becomes a brick. The heat needed to do this is pretty high, think pottery kiln temperatures around 1000 degrees.