Both the windows in the doors of my '86 Firebird are a sloppy fit and rattle, leak, and make noise at highway speeds.

It is getting too cold for working on the car now, but what is the most likely worn component that makes the windows so loose?

I can roll the windows down a couple of inches from completely closed and easily move them an inch in or out.

Before I tear the door panels off (hopefully in the spring) to investigate I'd like to hear some feedback on what is likely the culprit.

 

I've got the same problem. In my case, it's the felt runners in the inside of the door that have worn away exposing the metal beneath. It's also the ancient cracked, warped, and missing weatherstrip, and the slightly loose bolts on the door interior. I think you and I are in the same boat. We both need to take apart our doors and replace/tighten what we have.

 

Same boat alright.

For a second I thought you had seen my car.