For general information try Wikipedia article on Firewire (also known by its standard name IEEE 1394), or your favourite search engine.
man man displays manual page for the
man program itself, and the
/firewire causes the pager (the program that displays the manual page) to search for the word in the manual - which it doesn't contain. When you want manual on something else, call it with appropriate topic, e.g.
man sh to get manual for the shell (system command line interface). You can also try using the
apropos utility to search headings of man pages for a specific term.