Funding.
It seems to me their mandate is to make everyone's lives better. This looks like a project to define web standards for the future. It also looks like they've got enough to last them for a bit.
In this context, one of the problems that Mozilla has compared to Google is money.
Honestly, for all the money Google has, Google+ isn't very impressive.
Interestingly, about half of that 100 million comes from Google to secure their spot as the homepage and default search.