I didn't see this posted earlier, but I apologise if it's repetitive reading for anyone. It isn't entirely clear what time period these figures refer to, but I think it's just last weekend.
From Friday's Guardian (a UK newspaper) come these official box office figures:
Serenity = number 1
for the weekend of 7-9 Oct, it was appearing on 355 screens (which seems remarkably high, given that the sure-fire winner Pride and Prejudice - which Serenity knocked down into 2nd place - was on 411 screens)
it grossed 958,815 pounds that weekend (that's about US$1.7 million)
Serenity = number 8, down from number 2 previously
for the weekend of 7-9 Oct, it appeared on 2,189 screens
it grossed $5.35 million that weekend, and has grossed $18.02 million since release
PS the sci-fi club at my university is holding a Firefly marathon tomorrow. Several friends are going with me. I am determined to convert some of them!