I use Firefox and only Firefox. I have Internet Explorer 8 installed, but I am not using it. Firefox 3.5 all the way! It is faster and safer. I don't have to worry about security holes that IE has.
I hate to burst your bubble, but FF has always been rather secure for the same reason as Mac OS - It has a small percentage of the market, Although as you can see in this thread, it's userbase is booming. This means that it's finally a viable target for malicious hackers, etc. And it happens. Have you noticed how often FF is getting new updates, now? Most of them are to patch up some sort of exploitation.
That said, I do use Firefox quite a lot, along with Epiphany as my other browser which I am slowly migrating to. I actually find Epiphany faster for browsing, plus the boot speed is miles better, due to it's tight compatibility with the GNOME environment.
I've got to say, though. I gave up on Chrome after a few days back when it was new. Well I tried out the alpha version for linux, where Google strongly suggests that normal users shouldn't use it.
I gave it around 3 hours use, and I didn't bump into any errors whatsoever, although it seemed a little sluggish on the browsing side, but that might be due to it having its own cookie library and cache to fill. It certainly didn't feel like the unstable, developing software they think it is, and that's with all of the plugins enabled, which is what they say triggers the bugs. I'm hardly won over, but they redeemed themselves from the bad memories I had of it.