Customisability.
Whilst I like Chrome's speed, my problem with it is that it never feels like my browser. In Firefox, I have the amazing Awesomebar to help me find things in my bookmarks and history. Chrome's Omnibar is frankly no comparison - its layout is confusing and the algorithm is far inferior to Firefox's in terms of suggesting the right site.
I also have my range of extensions... I have Adblock Plus with the filters just how I like them. Chrome has Adblock, but where Firefox Adblock stops the browser from downloading ads in the first place, Chrome Adblock has to wait for the ads to appear and then hides them. This is almost as annoying as just having the ads! It also highlights the problems with Chrome extensions... the things which can be done with them are pretty limited, because they sit on top of the browser rather than integrating into it. I don't consider Chrome's extensions to be proper extensions at all; I'd call them "widgets" rather than "extensions", because they only change the browser in pretty superficial ways.
Firefox is also awesome as a development environment - there's nothing which compares to Firebug.
I see Chrome as a highly competent replacement for IE. It has a long way to go before it can replace Firefox. Chrome is a simplistic browser; it performs basic browsing tasks very well. Firefox is a powerful platform as well as a browser. I save far more time through Firefox's customisability that I would through Chrome's ability to load a page a fraction of a second sooner.