It's hard to pinpoint one thing. They're just much more usable and don't make you fight against it. It just gets out of your way. That's great UX design right there. Just measure the amount of key strokes, clicks, time and overall actions needed to do one thing in Chrome and the same in Firefox. You'll be amazed.
I would hope or presume that Firefox's tools eventually do get this level of polish, though. Chrome (and even Safari) has had quite a bit of a head-start, and neither of those were shining examples of great UX at the beginning.