Then they should have written "increased our market share again". Growth in corporatese usually refers to revenue or profit, which is not the main point here. Confusing choice of words.
And it's important to remember that increasing your userbase isn't enough to increase your marketshare — Opera, at least until Chropera shipped, was increasing userbase month after month, yet marketshare was falling. The number of people using the web continues to grow!
My understanding from reading Mozilla people, and from my own intuition, is that without significant market power for Firefox, Mozilla loses influence for all its priorities, including the open web, end-user control, etc.
Firefox's market share is very important to Mozilla, to everyone who shares Mozilla's priorities, and to everyone affected by them (which might be, simply, "everyone").