I'm skeptical this works. Many mic/headset speaker combinations have two plugs rather than one, so they won't work. A mic only, if it worked at all being plugged into a speaker plug, would most likely cause the internal speakers to stop working, which is counterproductive for Skype.

Even if you can find a mic/headset speaker combo with only one 1/8" plug that does work, what is the point. Using Skype with a WiFi only tablet without a camera is not going to be that big of a deal. If you have a WiFi connection you probably have other devices which will do the job a lot better than a Fire.