I switched a couple of years ago after I was sick of getting viruses and haven't looked back. I actually tried Opera first but I didn't like it (can't even remember why). I remember having to use IE 6.0 at work for ever, I was going nuts without the tabbed browsing. I don't even know how I surfed the net without it!
That's an interesting point about FF sucking up your memory. It's current using 110mb of ram right now and I've got maybe 6 tabs open. It's been running for a couple of hours. It gets up around 250mb if I leave it running all day. What's up with it always sucking my ram?