Since I have no official WotC capacity to give an official answer I can only logically speculate.
MCP, it grants a Stat replacement to the character. kinda of like a stat Boost, but permanent. You could equate it to a character with both Cunning Attack and Opportunist.. no matter who they attack, they will get one of the bonuses, and since they both boost the same stats by the same amount, its in effect, a permanant boost.
Now if a character, say an Aqualish Assassin were in a squad with Mas, Jabba, Crime Lord and Thrawn.
Now the AA would get the boost from Cunning or Opportunist, as well as the stat boost from Thrawn's Commander Effect.
And since MCP is an SA that gives a 'stats boost' then I would say that Fire Control would apply in both a Pre-MCP and a post-MCP situation.
A character with MCP and under the effect of Fire Control would need to copy another characters stats higher than its own, so it would need to copy a character with an attack greater than their Base + Bonus(in PROXY's case it would be +5 IF he were non-unique).
Furthermore, after the stats have been changed, then Fire Control would again apply, grant +4 to their attack.
Hope that helps