I just solved this problem. Before figuring it out, I had searched here looking for help, and all I found was others asking the same thing, with no answers. So now that I have the answer, I thought I'd post it here so that the next guy gets a hand.
This happened to be on a 1994 Ford Escort, but I'll bet it applies to other model years and perhaps even other Ford lines.
The rear view mirror is affixed to the windshield via a little metal mount. The mount is about 1 inch by 0.5 inch by 0.25 inch, and is glued to the windshield. The mirror asembly slides onto that mount point.
The trick is getting the mirror assembly apart from the mount point, whether the mount point is still affixed to the windshield or not (i.e. fallen off). You can try all you want, but it won't come apart. Well, I just tried harder and eventually forced it off. Of course, once it was apart I could see what the problem was.
In the bottom of the mirror assembly is a small square hole that seems to serve some sort of purpose. Well, it does. Inside the mount is a metal spring tab that acts as a lock to prevent the assembly from sliding off the mount point. I managed to just force it apart despite the locking tab. But the smart way to do it is to poke something small into that small square hole, like a small hex/allen wrench or a small screwdriver. You poke into the hole until you feel it hitting something, and then you push a little further to depress the metal locking spring tab. Then push the assembly off the mount point -- should come right off.
Additional keywords: removal, remove, rearview, stuck