Username or EmailPassword
For the people saying how they downloaded more than once, there are also situations where someone downloads it once and then distributes it to a number of users (think linux distros, company it, university it, etc.). Really there's no way to know how many people actually use firefox.
Also, on a firefox note, every site I use now works with firefox except for online billing with verizon wireless. wtf is wrong with them? Anyone else have success there? It's lame that I have to boot to Windows to pay my bill.