It's not easy because supporting open source contributors is hard, and Mozilla is trying to keep their staff focused on corporate and foundation priorities.
Even if users could directly donate money to develop Firefox, the implication is that any such donations should only be used for Firefox development, when in practice, shipping a modern browser in a way that is competitive requires an enormous amount of non-"Firefox" related work (safe browsing, sync, telemetry, marketing ,release engineering work, advocacy, documentation, etc, etc, etc). Earmarking donations for engineering work is kind of silly in the context of an OSS project designed to compete with some of the largest juggernauts in the industry.
My understanding:
I'm fairly sure they've said they are not allowed to direct money from the Foundation to Mozilla Corporation, and it kind of makes sense until you realize it is the coroporation that creates the outcome you want not the other way around.
Also - from my point of view it looks like the Foundation is overfunded the last few years while the guys who create Firefox are underfunded, but I'm not an expert so it might very well be more complicated.
I also don't want to say Mozillas management are useless: they somehow managed to land a huge deal earlier this year, but I will admit I sometimes find some of their decisions puzzling.