I made a bag for potatoes just winging it, and this was before I did much quilting. I had seen them in a store and figured I could do that only better
. I used Warm and Natural, and in all probability cotton-wrapped poly thread. It worked fine, but I saw absolutely no improvement over the usual microwave potato cooking, and so have just used it as a hot-mat on the table ever since. Is there any chance the batting in yours was Insul-brite, which is not supposed to be used in the microwave because it has metal?
I cook naked ears of corn under the microwave spatter cover and it turns out great.
In the future, if there is ever a fire in a microwave, you and all your family members should know that all you should do is turn it off and wait until the fire goes out. It will because there is a limited air supply in there. That's much safer than trying to move a burning item, and was mentioned in the instructions that came with my first microwave. Having a fire extinguisher in the kitchen is also a good idea, and your insurance company might give you a discount if you do that and a few other simple things around the house that can reduce risk.