How can I thicken/firm up this caramel?
The problem with quick caramel recipes is that they aren't reliable due to the variations of water content in the butter and brands of condensed milk. Butter can vary between 15-30% water, if your butter has more water the caramel will be looser.
The way to fix it it to simply cook it down longer. I personally think the microwave method is a bit gimmicky, all the opening and closing of the door and stirring doesn't save much time or effort. I use the same ingredients and cook them on the stove as it is the condensed milk which is the secret to the quick caramel, not the microwave. Cook it high to get the color, then low to thicken it until you get the right consistency. A wide pan helps with this as it increases the surface area for evaporation. It really doesn't take that much longer, just a few minutes in my experience.
If you are in a hurry reach for the cornstarch and add in small amounts of milk cornmeal slurry until you get the consistency you want. This will do the job but won't have the same strength of flavor as if you cook it down to thicken it.