As a for-profit corporation, Google by law has an obligation to provide the best possible profit to its shareholders, the "mission" you cite can merely be a tool for that for some time and has to be bent as far as possible or even broken when it has only a slight conflict with making profit for shareholders.
As a non-profit foundation, Mozilla by law has an obligation to stick by its mission or lose its status. There's no possibility to bend or break it and still comply with laws.
Therefore, I would not take Google's "mission" as much more than marketing but Mozilla's as a ground rule.
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