Is the Ford Thunderbird parts compatible with the mercury cougar?

Possibly. I have a 1995 Cougar with the 4.8L V8 and have used various parts from a 96 Thunderbird. My understanding is that, at least from 94 to 97 the 4.8l v8 engines are the same for the cougar, t-bird, mustang, Lincoln mark VII...

You can find a usergroup online for cougars and thunderbirds, they have alot of info!