It's just that chrome devtools are really, really well made in many dimensions. But keep in mind that Mozilla has been catching up a lot (impressive knowing the budget difference) both in devtools and under the hood (memory usage).
I heard it was an unmaintanable mess, that's why Mozilla added in-house devtools. I liked FireBug UX more, but I like that Firefox has metalevel coding/debugging capabilities out of the box (the smalltalk fan speaking).
