What I say might sound like blasphemy, but I've been pretty dissatisfied with Firefox with their last two releases. Ever since version 1.5, I've had to contend with a massive memory leak, and now, Firefox seems to freeze more often. While the memory leak could be solved with some quick hacks, Firefox seems less stable than it use to be.
I think the pundits are premature in saying that Firefox is an IE killer. If the developers of Firefox don't focus on basic stability and memory leaks so that their application is at least RUNNABLE, instead of just adding features, there's no doubt in my mind that Microsoft will pull through with their IE.
Microsoft has survived this long, mostly because they've made less stupid mistakes and never underestimated the competition.