And anyways, I think by "win", they don't mean "make Chrome irrelevant". Competition drives innovation, and the Chrome team probably want to steal good ideas from Firefox, find the feature that will put them back on top, etc... They are likely to get more resources and a more interesting job than by staying alone on top.
The best part is that Mozilla and Google are not really competitors when it comes to browsers. Google doesn't make money from Chrome, it is just a gateway to their profitable services. But Firefox works too: and ad impression on Firefox is not worth less than an ad impression on Chrome.
Why aren't they working on Firefox then?