As much as I loved FA in the old days (later releases just seemed to make the game successively less fun in my eyes) I'd rather see something closer to a FA2 than a FA:S- ultimately, remaking much of the game from scratch to take advantage of the Source engine's capabilities. Unlikely to happen, mind.
I love Firearms!. I am/was like the biggest fan ever of that mod. I played it for roughly 2 years until I finally moved on to other games. I got really good at it, csports.net (when HL was still on WON) said I was in the top 10% of the world for that mod. I loved that sometimes teamwork was needed while other times it was just run-n-gun. If you were skilled enough, you could make any weapon deadly leading to tons of combinations of armor/weapons. The sheer variety!!
I realllllllllly hope they port to HL2 or at make Firearms : Source.
I tried FireAarms once, didnt like it all that much. I'd try it in souce and probably feel the same. No harm done with it though. Aslong as it keeps some people happy!