I'm kinda this way about squeaky brakes,
, they don't necessarily mean there's a problem, nor do they mean that cheapie pads have been used. The last time I had my brakes done (and rotors, too) with good quality pads (may have been Pagid, I can't recall) I was getting some squeakiness that was rather annoying, but entirely benign, until they were run in for awhile.
I doubt (or don't want to think) that a major company like Firestone would use substandard components, although I suppose they could have put in a harder pad than you need, which could account for the squeaking.
But you're also concerned about a metallic sound... I wonder if a sticky caliper might be the culprit, or to approach this from the simplest perspective, maybe a good blast of compressed air might blow out anything lodged between a pad and rotor.
Wish I could be of more help...