As a financial supporter of the Mozilla project, it would be stupid of me to use another browser. Additionally, I have been using Mozilla products for like 4 years now, and they have grown on me. You know how it is, sometimes you just feel incredibly comfortable with something, it feels like home. Opera is neat, but it doesn't really do anything that I am too impressed with - your specific complaints about the download manager have never been an issue for me, since I use a T1 line most of the time.
My browser sitch right now is crazy - I use Safari when I'm on the Mac. In Linux, I use SeaMonkey for regular surfing and Firefox for porn browsing (some of those extensions are unbeatable). Occasionally, just to make sure it's working, I run DeerPark too.
Eventually, if I come up with a neat idea, I would like to build some extensions for Firefox. It's nice to know that the license allows me to do that. But that's really the only way in which the license has a "real" effect on my choice of browser (it has all kinds of "fake" effects
).