Easy account sandboxing and profiles would be the biggest for me. Both Chrome and Firefox have this in some way, but it's still rather primitive.
I think the bigger issue for most people are the switching costs. I don't even know if Firefox is better than Chrome, and honestly, I don't care, because I don't want to put the effort into switching. The only way to get me to switch was if Firefox had some absolutely huge advantage over Chrome that would make switching worth it. But I can't even think of what that would be.
The multi-profile functionality in Chrome is just so much more usable than Firefox. Yes you can hack things together with command line arguments and shortcuts, but its nowhere near as seamless.
I think the bigger issue for most people are the switching costs. I don't even know if Firefox is better than Chrome, and honestly, I don't care, because I don't want to put the effort into switching. The only way to get me to switch was if Firefox had some absolutely huge advantage over Chrome that would make switching worth it. But I can't even think of what that would be.