I'm fairly sure that's for donating to Mozilla, where the funds go who knows where (kidding, it goes to the executives and marketing).

Is there anywhere I can donate to Firefox, specifically the development and the maintenance of the browser itself, and only the browser? Maybe donating directly to developers working on Firefox would be the best approach here.

Nope! In fact, last I heard, donations to the Mozilla Foundation could not be used at all for the browser, which is developed by the Mozilla Corporation.

That's for donations to the Mozilla foundation, they aren't used to fund development of Firefox. Mozilla corporation and Mozilla foundation are distinct entities.