An interesting discovery yields that Google is collaborating with the Mozilla® Foundation in the development of Firefox. Their specific requirement of several years C or C++ programming experience and the "ability to quickly and efficiently learn the intricacies of a large scale code base" can lead to the assumption that they really plan on getting their hands dirty working on the guts of the program, and want someone who is up to the task.
While this may seem like an initiative on Google's part to get involved in browser development, possibly to begin work on their own browser, that seems unlikely. Google has been very involved with Mozilla for almost a year already, and has already expressed that they don't have plans to release a "Google Browser". Concerns over whether Google could have a corporate say in Mozilla decisions can be combatted with the fact that Google hiring an engineer to work on Firefox is likely not an official collaboration between Mozilla and Google. Rather, its probably Google 'helping out', and offering Mozilla the opportunity to implement improvements that Google engineers make in the software. So the picture looks the same as in the past; Google is interested in promoting open-source software, without worrying about what's in it for them.
The engineer job is based in Mountain View, California where, conveniently, the main offices of Google and the Mozilla foundation main both reside. I'm sure the close proximity, along with Google's active involvement in open-source software, helped the relationship of these two groups, especially when you consider previous collaborations between them, such as Google Adsense's referral program having offered Firefox as the first non-Google product available to promote in exchange for a dollar (USD) per download for the webmaster. (Google Adsense is a service provided by Google for webmasters to place Google-provided advertisements on their pages and, of course, get paid in return). I can more than imagine that having these two of the greatest software providers working together will lead to software quality beyond expectations.