Humm. My experience are completely opposite. In my experience firefox breaks down quite fast and chrome has always been many orders of magnitude better than any browser I've tested (firefox, (safari), opera, IE). I have about 500 tabs in chrome now, never been close to that with any other browser, granted the computer is barely usable now as chrome takes pretty much all resources so I'm working on closing some down :…
Firefox 7 and 8 (both in development) are much better behaved over long sessions. You should give one of them a try.
Chrome on the other hand has always handled 300+ tabs without blinking. Closing and restarting it with that load takes about 20s to be usable and a couple minutes for all the content to be loaded — basically limited only by my internet connection. It crashes about one every three months, ironically only when I tell it to quit.
Firefox would have to now be three orders of magnitude better in these respects in order to have caught up with Chrome.
P.S. Mozilla's messaging with "try version 7 or 8" is fucking abysmal. Seriously shut the fuck up about version numbers if you're going to increment them so often. Don't copy Chrome's development cycle without also copying their release structure: normal, dev, canary.