Firstly, out of all the other browsers that Mozilla would want to have take share, Chrome is the obvious first choice due to shared ethos and belief in the future direction of the web (e.g. WebM, WebRTC, WebGL etc.). The last choice would be IE, which is where Chrome is mostly taking share. Chrome's gains are therefore only a positive for Mozilla.
It's obviously not only a positive, but they're aligned on a lot of things. Still, in some ways Google is trying to push the web towards whatever they like, whereas Mozilla tries to find out what would help the web move forward. For example, Mozilla likes WebM because it's an open standard, but they don't think WebP is worth the effort of yet another image standard.