I re-watched Firefly very recently, and was wondering just this question. I saw Serenity back in the day, and my reaction was “meh” for two reasons:
First, they had to re-cap everything that had happened in the series (or at least, re-introduce all the characters) so that newcomers would know what the hell was going on. I understand why they did this – probably it was something in between the assumption that a film audience would expect it and the demands of the studio. I don’t care. There’s a weird intimacy between the makers and the fans of Firefly that is betrayed when they do things like that. And we’ve all heard the phrase “in medias res”. It all could have been done more artfully.
Secondly, I recall that it was a lot more graphic than the series was – I assume in the context of the reavers storyline. I’m not crazy about that kind of stylistic leap. As if either the writers felt too constrained while working on the series, and now they’d been set free, or else it was felt that a theatrical release demanded a bigger punch. Anyway, I’m not averse to violence in film or on tv, but I don’t like jarring switches between the universe my characters live in on tv and the one they live in on film. It takes me out of the experience.
So, that’s the reaction that I recall having several years ago. I will be watching it again to find out if it still hits me the same way. Maybe I’ll find that my reaction will be completely different, now that I’ve seen the series a few times over.