I'm going to be a bit controversial here...
For the casual (non-technical) user I would never advocate anything other than Firefox. There's no doubting that Firefox is more secure by default than IE will ever be, and it's certainly no stretch to state that there are always going to be more risks involved in running IE7 - if only because it is so tightly integrated into the rest of Windoze, and because it will always be a target for the maliciously-minded anti-Redmond brigade.
However, for the 'techies', I can't see there's that much difference tbh. Yes there are some cool extensions for Firefox, but i can live without them.
What is pissing me off about FF the most is the fact that there is STILL a massive problem with the cut & paste functionality with the browser, which has NOT been fixed, despite seemingly every new release trumpeting that it HAS been fixed. This bug drives me insane - I've never been a fan of tabbed windows and often open separate instances of FF - but as soon as I do this, its a race against time before the copy & paste bug appears. This is an almighty cock-up on Firefox's part.
Rant over.