If you were designing a core business intranet app for a small business, and wanted it to be as responsive-feeling as possible, where the staff are indifferent to being stuck with a certain browser, would you design for Firefox, Chrome, or test more widely than you need to just to avoid lock-in? Are there other factors you'd consider before placing all your eggs in one browser basket or not?
For instance, does Chrome have any speed-related features that other browsers lack that would need Chrome to be targeted in a cross-browser-unfriendly way, and if it did, would it be worth designing around them?
What is the business' IT policy on browsers? It seems like this would determine what you write it for. Why write it for either, if every computer at your client's company has IE6? Lock-in is not a problem for a company as they can dictate the policy on what people use and must have on their computers.
Speedwise you're not going to find significant gains choosing one over the other unless you're doing specific tasks that play towards their strengths (look at the range in benchmarking for instance, some google tests have chrome hundreds of times faster, where as others have firefox beating chrome).
Ideally, you'd want it to be agnostic, but if you had to choose one though, I would go with Firefox, as it's a more mature browser. I'd be reluctant to target the app for a browser that is only a couple months old, especially one that just hit 1.0.