There's plenty of massive open source projects that have enormous install bases. Linux would be pretty high up one in that list. There are far more Linux servers than desktops so don't get hung up on how many people have Linux installed on their personal computer. There's millions of servers out there running Linux.
I'm aware of all those Linux servers chugging away out of my sight, and I've read some ATM's are also running Linux (For a long time they were mostly running IBM OS/2 - and some still are, while many run customized version of Win XP Pro).
However I was specifically referring to consumer-installed (including pre-installed Linux), where I believe FFox would far outrank Linux in terms of number of "elective" users. Mozilla claims 450 million users and between 20-24% of browser use world-wide. Desktop Linux is estimated at between 1-4.8% of the PC market by various metrics according to WikiP.
And I specifically excluded Android as the parts consumers mostly see are Googleware - and all the skins, overlays and forks piled on by OEM's.
It is true, however, that Mozilla depends on corporate sponsorship. As detailed here: http://en.wikipedia.org/wiki/Firefox#Affiliations with the bulk coming from keeping Google as their default search engine.