Firefox has been improving noticeably faster since Chrome came into the picture. Not just lately either. FF has gotten much faster, much less memory hungry, and added a lot of new features (many inspired by chrome).

I find I use both regularly, and which I use by default changes often, depending on the circumstance. E.g. they're both pretty fast these days, but on different things. FF seems more memory efficient overall, but chrome's per-tab-process thing makes it easier to manually reclaim memory by closing a tab (the per-tab-process also probably makes chrome less able to share memory between tabs, so there seems an inherent tradeoff, and explains FF's win here).

So it seems a virtuous cycle, and chrome's to be lauded not only for a great browser, but for heating up the browser race in a way that seems to have resulted in every browser improving.