Let's see -- one major flaw in a work-in-progress as opposed to a production model that has had multiple flaws/exploits/problems/et.al.
I'm not saying that FF is bulletproof, but overall, it's a better browser than IE is at the present time. And until MS begins to go back into active browser development instead of the ongoing stagnation they've engaged in, then Mozilla & Opera will remain head and shoulders above them in the development department.
I use Firefox at home and many other open source tools, but this is a cop-out. Mozilla has been under development long enough that the "beta means never having to say you're sorry" excuse doesn't cut it.
There's a reason most open source tools never leave beta stage or get superceded by newer beta versions. Not having to accept responsibility for a finished product is one of them.