Let's not talk about privacy (because there is no point in talking about it: Firefox is eons more private than Chrome - or any of it's based browsers - can ever be)
About security: Chrome has a biggest workforce, yes. but let's think about this a bit...
First, let's not forget that chrome is also a bigger target.
let's imagine this:
Consider that 90% of the users worldwide use chromium-based browsers, and you are an hacker who wants to steal peoples data or access their computers.
Would you bother targeting 10% of the users. Or would you just go after those 90%???
now add another detail into that thinking:
people who use Firefox are mostly techies, people who know about computers, gnu/linux users, developers, more security-conscientious users, people who actually know and care about the tech that goes bellow, people that knows what's happening in the IT world, and people that simple don't go with the flock without studding it's path first... now... would you really bother targeting those when you have 90% of people - where probably 85% don't know anything about computers or just don't give a #$%& about it???
Would you go easy bait, or would you try to outsmart those who might be at the same level you are???
(sure, there is always exceptions!!!)
but then again... maybe that's just me...