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If you do not use them often, pull them off and stick some paper towels in the hole and jam them back on. A better solution is to get velcro tape the same color as your headliner and stick them up as long the tape (headliner side) is attached to something solid like a thin piece of plastic and attached to the headliner by rivets. If you just stick the velcro to the headliner it will rip and pull away from the cardboard.
Okay, short of putting rivets in your headliner, lets solve the problem. If you take the sun visor off you will find a spring. With time and age the spring wears out. You can find new replacements almost anywhere (ebay has them cheap all the time) and you just replace them. Your local Ford dealership will have them (or can get them) as well. While I am an avid user of Velcrow, I don't think riveting something to your headliner is the answer. Sorry if I offend anyone.
Try this fix I learned on another site. Pull the visor portion off of the rod on both sides. You can do it, just grip it, gently pull while swiveling it. Then switch the rods side to side. (Mount the drivers on the passenger side and vice versa.) You'll just have to turn the rods 180 deg in the mounts. Then push the visor portion back on. Voila, no more floppy visors.