In "Fahrenheit 451" is Faber a coward?I know that Faber was initially afraid to hide the books in the firemens' houses when Montag first suggested...

Faber calls himself a coward when he is speaking to Montag, even before the planting books scheme came up at all.  He states,

"You are looking at a coward.  I saw the way things were going, a long time back.  I said nothing...and when they finally set the structure to burn the books, using the firemen, I grunted a few times and subsided...now it's too late." 

So, back when the trends started changing to where firemen were burning books, Faber was a coward because he said nothing about it; he didn't fight, he didn't start a resistence movement, and he didn't rebel.  He cowered at home, doing nothing, in order to save himself.  Also, when Montag met him at the park the first time, he was super suspicious and in denial about anything in regards to books; later, when Montag calls him he also cowers and hangs up the phone denying knowing anything about the bible.  So, Faber has had a history of backing down and playing quiet for survival's sake; he feels a coward for it.  However, it is that background that primes him perfectly for action when Montag steps into his life.  He is ready to act and vindicate himself from his past acts of cowardice.