Google certainly values the eyeballs they get via Firefox, but Firefox could disappear tomorrow and that barely affect Google one bit from a pure "eyeballs" perspective. How much traffic would Google lose? People would just use those same Google services through Chrome, Safari, native apps, whatever.
So, I believe Google funds Firefox mainly as a hedge against monopoly charges.
If Chrome ever dominated the browser market entirely with 95%+ marketshare like Internet Explorer back in the bad old days of IE5/6, they'd be painting a large target on their backs for various governments to step in and make life difficult.
If we think about it, Chrome/Chromium's current ~70% market share is pretty ideal for Google. They get to kinda sorta mostly control the web, without a bunch of pesky governments on their backs.