What does "dir firstp.*" have to say? If Nasm can't open your input file, chances are it's not there! Either it wasn't saved *where* you thought, or wasn't saved as the *name* you thought. Or you didn't tell Nasm right (if your editor saved your file as "firstp.asm.txt" or something, Nasm will assemble that, but has to be told the right name). (What build of Nasm are you using? For Windows, you probably want the Windows build. I like the djgpp build, 'cause it'll run in Windows *or* dos - but it *won't* do long file names. The Windows build is usually named "nasmw", which is what makes me ask...)
Or, you've discovered a bug in Nasm... but I doubt it. :)
Best,
Frank