A big part of it is extensions, I have a handful of extensions that I use on a regular basis, and Firefox is the only mobile browser that actually supports extensions.
Another part of it is that I just trust Mozilla more. Both in terms of conflicts of interest, and just plain fixing bugs. For example, Safari on iPod touch has been unable to access the microphone for years, even though Safari on iPhones can.
I will concede your point about security. I think that iPhones are probably more secure than Androids. But, I feel that the security of my Android phone is sufficient for my needs.