The reason Firefox has been downloaded 75 million times:
Every time they find a security update you have to download the whole program.
There are actually 100 users who have updated 750,000 times each. I know, I downloaded it again on all 5 of my computers last night. That's 5 more downloads!
The nightlies already have the binary patch system enabled, and it should make it into the next release, so that'll be the end of that.