Hi.
Well as an example, my forum account here is "kiw" (even though it has an alias and displays as "BFM_Kiwi") My xfire account is "kiwnz", also aliased as "BFM_Kiwi". My halo profile is BFM_Kiwi. None of the three have anything to do with each other really though.
If I use xfire to join, I believe it uses whatever profile I last used in halo. So if my sons were playing and I join from xfire I might join the halo server with their profile. So my xfire account could be "fred" and it wouldn't matter, if I only had the profile BFM_Kiwi setup in halo, that's how halo would start up if I joined from xfire - not as fred.
So the only thing that matters really are the profiles you have set up in halo.