The only thing that I have noticed is when doing the upgrades it prompts me to run Firefox as an administrator so that the update can actually be installed. Before, if I didn't manually run Firefox as an administrator the update would silently fail but you wouldn't really notice. I keep UAC off by default so it was never really a problem for me anyways, but for others this is a really big deal especially if you are using nightlies that have a new version every day.
