Firefox already uses bsdiff to generate binary diffs for update distribution. Courgette looks like a promising way to shrink those updates even further. (Details in the discussion linked from: https://bugzilla.mozilla.org/show_bug.cgi?id=504624)
I can't find a reference, but I think the "one billion" count only counts full downloads; updates installed via "Check for updates" aren't included in the count.
The latest trunk build automatically downloads and installs the newest nightly build every day.
I've been using firefox since version 0.5 (2002) and I've probably been running the latest-trunk for half of that time.
Conservatively, I've downloaded firefox > 1000 times. Just sayin'.