There's no short answer to this question, really, since neither browser is clearly better, so instead of a straight out "Use X" recommendation, let's consider the pros and cons of each.
First, in our regular web browser performance tests, Chrome has regularly beaten up the competition or come in a very close second in most categories, while Firefox rarely leads the way. That might seem like a pretty big deal, but most browsers actually perform really well these days, so the fact that Chrome beats Firefox in most of our performance tests doesn't mean Firefox is extremely slow—it just means that compared to Chrome, it's not the fastest.
But that brings us to another important point: Bloat. Despite the fact that Firefox generally does pretty well on memory use tests (strangely enough, it regularly wins that test in our browser performance tests), a lot of Firefox users are increasingly frustrated with slow-downs caused by what they see as browser bloat. It's not at all uncommon to see a Firefox installation eating up more than any running application on your system, and while the memory consumption itself isn't that big of a deal (Chrome eats a lot of memory, too), the high memory usage is often accompanied by serious browser slowdowns, which is a very big problem, and one that, anecdotally, at least, we hear from tons of Firefox users and very few Chrome users. (Presently company included.)
So what keeps Firefox users coming back despite Chrome outperforming it on Windows by most accounts? One of the biggest issues is extensions. Firefox's extension ecosystem has long been a selling point of the popular open-source browser, and if there's anything you wish your browser did differently, chances are there's a Firefox extension to address it.
Chrome has its own flourishing extension gallery, and while it doesn't have close to the extension support that Firefox does, every day more and more really popular extension functionality makes its way to Chrome. Want to sync Chrome up with your existing Firefox bookmarks? You can use Xmarks for that. (Or you can use Chrome's built-in bookmark sync tools if you prefer, though this editor still dearly misses keyword bookmarks in Chrome, despite the workaround.) Want to securely sync and automatically fill your passwords? The LastPass extension, which we recently raved about, can handle that at more. Even the must-have for web developers, Firebug, has made its way to Chrome in a very functional lite version.
If you use a ton of Firefox extensions—especially some more obscure ones—chances are you may not be able to find replacements for everything in Chrome. But if you only use a few of the more popular Firefox extensions, odds are very good that you could transition to Chrome without noticing a difference.
On the other hand, a lot of people shy away from Chrome specifically because of its affiliation with Google. Those who believe Google already has enough of your personal information might prefer to stick with Firefox just to temper the growing piles of information Google has collected about you, anonymously or not.
At the end of the day, both Chrome and Firefox are excellent web browsers for Windows, and you can't go wrong with either. From our perspective, more and more power users seem to be swapping happily to Chrome from Firefox (several Lifehacker editors have made this switch and haven't looked back), and their switch has been spurred mostly by the bloat issues mentioned above. If you've been frustrated with Firefox, we'd recommend kicking the tires and Chrome to see how you like it; you can always switch back if you decide your heart lies with Firefox.
If, on the other hand, you're perfectly happy with Firefox, we don't see any reason to upset a good thing. (If it ain't broke and all that.)
Which Should I Use on My Mac: Firefox or Chrome?
The answer to this question is very different from the one above, but luckily it's much shorter. If you read the Windows section (and you should, because a lot of it still applies), you know that Chrome has a lot of great things going for it. Chrome is snappy on OS X, the same extensions that work for Chrome on Windows work in OS X, it isolates processes so that one crashed tab won't bring down your browser, and so on.
Unfortunately, in our experience, Chrome for OS X is still much too young for full-time adoption. Remember, Chrome for Mac launched quite awhile after the Windows version and has been around much less than a year overall. Like Chrome on Windows, it shows a whole lot of promise, but it's also prone to the occasional non-responsiveness and other bugginess that we aren't used to and probably shouldn't be seeing from a prime-time browser.
While we'd love to say you should give it a try full-time if you're not happy with Firefox on your Mac, we can say from experience that it's probably not ready for you to do that.