User-base has no importance, dev-base is what matters here. If Mozilla sends a letter to developers, chances are those people already stumbled upon MDN multiple times when they looked for answers to web development related questions.
If there's no law saying they have to support those 3% of customers, and supporting the 3% costs more than the 3% bring in, then yeah they won't bother.
I also use it for developing and use its developer tools. If it works on Firefox, there's maybe a few CSS tweaks I need to make when I test on other browsers
