I checked the AC-Delco Durastop catalog today at work. This is what I found.
I was looking under camaro and I assume that the firebirds are exactly the same. The third gens came with two different master cylinders that the book listed.
One came on the 84-87 with rear disk and on the 88 with rear disk, but not the 1LE. This master cylinder had bigger bores than the other one. (25.4mm x 36.0mm) I assume that the smaller diameter is for the front brakes. The Durastop part# was 18M314
The other, "better" master cylinder was equipped all 89-92 third gens, 84-88 with rear drum, and 88 with the 1LE. This master cylinder had a smaller bore. (24.0mm x 31.75mm) The Durastop part# for this master cylinder was 18M313
If anyone needs any more info, ask and I'll see what I can dig up.

 

That stands to reason. Because when I removed the larger bore master cylinder from my car, and installed the smaller bore M/C. I noticed a dramatic improvement in brake effort/performance. I used a J50 M/C and booster from a 1986 Camaro.