I don't know how to explain it. I think I just like the interface.
I've tried Opera, Chrome, Safari, Internet Explorer, Kazehakase, Galeon, Epiphany, Dillo, Camino, and Lynx.
Even though Opera and Chrome and the fastest, I still prefer Firefox, regardless of whether I'm using Windows, Mac, or Ubuntu.
As for security, you can't get much better than Firefox with the NoScript extension. Of course, most people will probably find NoScript too much of an inconvenience, they'll visit fishy (or phishy) websites and click on anything, so I agree wtih your assessment about security threats growing as Firefox gains marketshare.
Ultimately, marketshare brings security threats because it brings more stupid users who can be tricked into installing anything.