They're both fine in usability, but I'd rather be using Mozilla's browser than Google's or Apple's. It seems like Mozilla has better incentives than Google to improve the World Wide Web as a whole rather than just its own WWW properties.
I still use Chrome for JavaScript debugging. I'm more familiar with it and the tools are better. I expect to switch to Firefox for that when their profiler improves and when they start putting the full stack trace in the console.
Edit after actually reading the article: it looks like they have done the stack trace thing in this very release!
Some parts of the Chrome Web Inspector still outmatch Firefox (and Safari), esp. the profiling/heap view parts. Right now I switch back to Chrome when I need this functionality. Firefox’ web inspector feels a bit faster to me overall, though.
With Firefox’ new design in the nightly and the recent speed upgrades I don’t see a technical reason to prefer one or the other in many situations and that’s a very good thing. Welcome back FF!
My reasons are pretty trivial, and I'll readily admit Chrome can be better to work with in certain dev cases, but I like being able to customize the UI. (I think the back/forward buttons belong to the right of the address bar where they're easier to reach with a mouse and I like having an RSS button, dang it.)
I really like it. There's no noticeable performance hit, but I'm definitely finding it more stable. My favourite recent feature addition is the ability to pin add-on shortcuts to the new Firefox menu, which I'm finding a massive improvement over using the add-on bar/ramming stuff into the same toolbar as everything else.