Unable to make Firefox open Ack's output

Answer 1

Firefox takes URLs on its command line; open takes files. ack's stdout is not a URL (or a file). Instead, you'll need to write ack's output to a file, and have firefox open that.

ack-grep -a NB > ~/ack-out.txt
open -a Firefox ~/ack-out.txt

I'm not sure why you want to do this, but that should work. Note that you probably won't get color this way; ack does not write HTML AFAIK.