Yes, the book has to have the '1' in the number line. The reason that the true 1st printing's of the Firestarter paperback are so hard to find is because most of them ended up being part of the 'boxed' set that included The Dead Zone. I have both the black and white cover versions and it took me 20+ years of haunting used book stores and thrift stores to even find those!! Also, the true 1st printing has on the cover at the top, directly underneath the big red '1', 'From The Author Of The Dead Zone' instead of 'FromThe Author Of The Dead Zone And Cujo'. If you know this it will save you some time, so you won't have to open each copy you run across to look for the '1' on the number line.
Another one that's kind of hard to find with the #1 in the number line is the paperback of Christine with the photo of the car on it which would be the movie tie-in version. Most of them start with a #2. Something else to keep in mind in your travels.
I do have a first/first of the Firestarter paperback with the statement on the white cover "From the Author of The Dead Zone and Cujo" on it. It's the only one I've ever seen. I paid $47 for it a few years ago.
John