Username or EmailPassword
I've downloaded it once off the website and installed that on four machines. Three others have it installed via package manager in Linux and probably haven't gone near their server.
This only covers about three users, but still it's only one download.
All in all I think it'll average out; some will download it more than once per user, some less. I prefer to focus on the fact that it's been downloaded 25 million times; that's a bloody impressive number and shows that the Firefox team have done a damn good job, regardless of whether there are 24 million people using it or 26.
Would be interested to know how many of those 25 million have been downloaded to somewhere at microsoft.com :-D