I don't know the specifics for Firefox in particular, but update checks are classically staggered over at least a full day if not longer. You don't want every user of your product to hit your download servers at the same time.
I don't think that load on the download servers is an issue now. CDNs do a great job of distribution the load. The concern is more that some bugs (especially GPU related) only show up "in the wild". A slow rollout limits the damage if there is a show-stopping bug in the release.
