What if FF changes that in the future? What if a better search service appears, FF recognizes that and switches the default search provider? Lots of $ will be lost. Open source may be open, but there is still a small group of people in control. Google cannot directly control FF.
The thing is... the better search provider cannot be as rich as the richer ads/search/docs/e-mail provider.
Another thing is...Google understands why they overtook Yahoo and all the other web apps are there to leverage any loss from a marketshare perspective. I'm not saying Google's perfect (like how they miss out on Buzz or how Wave and Notebook was put on hiatus) but the FF situation even way back benefitted Mozilla more than it benefitted Google pre-Chrome.
1) Let's not forget that Google's primary cashcow are ads. Let's not forget that FF's primary user are Adblock users.
2) Google's marketshare didn't increase from being the default provider. Just see any browser that tried to make a different default browser: people end up returning to Google.
3) Google has secret blackmail type of antics. Just ask the problems Opera tend to have with fixing gmail. It's always Opera fixes things, gmail ends up working, gmail ends up not working, users complain, Opera contacts and fixes things, gmail re-fixes itself, gmail breaks again. Meanwhile, google services mysteriously works better for FF. Yes, even better than they work for Chrome.
When Chrome was released most of the blogosphere were talking about how Google screwed Mozilla and not the other way around because there really was no danger for Mozilla to drop Google anytime.
So, Google is not trying to take over the world, they are just trying to control the money pipeline leading to them. Having its own browser is logical.
If you control the world, you control the pipeline. Google wants to take over the world of search. Mission mostly accomplished. Now Google wants to take over the world of cloud applications.
Having it's own browser was illogical in terms of competing as a browser. It's been mentioned often times, Chrome does not have a bigger marketshare of users than Firefox. Chrome winning the browser marketshare is not only less profitable, they have a less than optimized browser that barely fits an alternative need on it's own.
On the other hand, Chrome "linked" to Google's OS allows Google to beta run most of the major problems that can be related to a WebOS. Allows Google to train it's customer to see the most often seen interface of their WebOS.
From the get go Chrome had no interest in competing with the browser battles. Their aim was much more grandiloquent. Aim at the heart of Windows marketshare by attacking it's weakest (and at the same time most exclusive points) - Office and IE marketshare.
Take those away and you can introduce an operating system that can leapfrog through most of the Linux and OS alternatives. Then once you gain a share of the operating system users, eat away at the remaining pieces of the prime competitor known as IE and Office. Once that taken care of, it doesn't matter if users prefer Firefox over Chrome. It will be installed in their WebOS which coincidentally will be syncing with Android data and you could potentially have the first dominant desktop-mobile-server-cloud-TV dominant service in the entire history of the internet which coincidentally combined with their improving offering of Maps/Images/Street Views/etc. services allows them to be the premier directory of everything and anything to the point that only services like Facebook would have their own marketshare as users are uniquely loyal to those services no matter how confusing, poor, private breach those services are. Every other service post-that type of monopoly and they could just slap a super search engine storage to their existing infrastructure and compete with any threat be it the likes of it coming from mobile/cloud/desktop/etc. (Again not saying their execution of their plan is perfect but just trying to steer any notion away from the idea that Chrome is a defensive move "in case" a browser switches to another search engine. That could almost count as historical revisioning even though history hasn't happened yet. Think Big, Not Small. The Google Way. Happened with Gmail. Happened with Docs and Spreadsheets. Happened with Wave. Happened with Ads. The smallest well known things Google have done - they often abandoned or mostly weren't updating post-release anyway. Chrome is the exact opposite of that and you can see it not only from the money being shelled in but for how early the first full version of Chrome came out of beta. None of those are actions of a company with an intent to just create a browser as a "back-up" utility.