They don't, they are livebearers, no need for a bubble nest. If they're hanging at the top you may have low oxygen levels in your tank. How is your filtration and what is your temp?
I have a 55 gallon tank with two aqua tech 30-60 filters and a bubble stone. The temp is 77 degrees. I keep checking my water, no ammonia or nitrites. Nitrates are around 10ppm. She seems happy. is zipping around the tank between bubble blowing sessions.
I only have two mollies, and I notice they always seem to doing some odd thing or another. As soon as I start to get concerned, they never do it again and move on to doing something else kinda-sorta strange.