Well... first, put the weed into a cup, put some water in it (about 1/2 of the cup), and microwave it until the water evaporates completly. It will be enough to decarboxilate the weed (what happens when we put it into the oven). If there is some material glued (cant find better word) in the internal walls of the cup, scrape it, for avoid any waste of THC.
Then, use this weed in the firecrackers. Put it in the nutella, and microwave just the time needed to make it. As you already decarboxylated the weed, 1-2 minutes will be enough. And there is no danger of evaporating the THC in the microwave, as it only reaches temperatures near 100C (212F), much lower than the boiling point of THC (>200C).
Ive heard is not nice to put any metal into the microwave. Ive heard there is a lot of sparking. I would like too much do make this experiment, but my mom never allowed to do it with her microwave...