^True, it's not a blatant contradiction, but I think it demonstrates an alteration of creative intent, a change to the subtextual foundations of the 'Verse. One of the primary rules of writing is "Show, don't tell." Firefly
didn't tell us that FTL didn't exist, because it did better -- instead of clunky exposition, it demonstrated it through story. All long-range communications were by mail or recorded messages, implicitly demonstrating that FTL didn't exist. Sure, you can justify a retcon by pointing out that they never said it didn't exist, but it's still a retcon, just as much as the change to the story of Simon's rescue of River was. And I find it a hard retcon to swallow because it's hard to believe that if they had such a useful technology as FTL communication, they wouldn't have used it routinely in the show.
One possible rationalization for the change is that Serenity
wasn't equipped with FTL comms until the movie. We know the ship got some upgrades between show and film -- a new hover-Mule, a revamped cargo bay, more video screens and fancy equipment in the cockpit. The novelization of the film says they bought the Mule with proceeds from the sale of the Lassiter pistol; I figure they revamped the ship using the same funds. Perhaps, if they were finally able to afford such upgrades, they were able to afford an FTL radio for the first time. (Although that doesn't explain why the primary buffer panel was still six months overdue for maintenance at the start of the film.) But I consider that an imperfect fix, since we never saw any indication that anyone else
had instantaneous communication either.
Plus, FTL communication is simply less believable than the alternative. I have less trouble with a character retcon, so long as it's reconcilable and the character's revised history or behavior is believable. The FTL retcon may be reconcilable, but it makes things less believable than they were.