I have this problem any time there is frost worth scraping on the car. Last year I was real good about storing the car for the winter. I had a few days this year when our daily driver was down and needed to use the Mustang. It was not snowing or anything, just frost from the condensation in the morning.
Anyway, I can open the door but closing it is a nightmare. You have to let the car warm up so the window track/weather strips will unfreeze. I did a search here and did not find it, but I know a lot of us had this issue. I remember the one suggestion was to push the window inward while trying to close it. That was fine if you were parking but near impossible if you were going to drive. The other is hold the power button down and try to put a little weight on the top of the window to free it from the ice.
It has happened a few times and made us late for work.
Has anybody found a fix besides moving to someplace warm?

(pperin)