Well, Firefox's architecture is currently being completely thrown over for Electrolysis, they are heavily working on their implementation of WebExtensions, and Servo+Rust are starting to take off at the moment, too.

Other than that, Thunderbird could certainly do with a bit of cash and Firefox OS is also currently being rolled out on a few TV sets, so could certainly use some financing to fix up initial bugs.

And if they really have nothing else to finance, they often also just redistribute money to other smaller open-source projects...