Actually you can't. You can donate to the not-for-profit Mozilla Foundation. But Firefox is owned by the for-profit Mozilla Corporation (which itself is wholly owned by the foundation). The Foundation does "other stuff" in addition to directing the Corporation to develop firefox (what exactly? it's not clear to me). It's managed like a single entity, but the org is broken up like this for some #bigcorp reason (taxes, control how profits are allocated, or some such). Not to drag on it, do business however you want, but my point is that you can't directly donate to firefox development because you have no say on how the Foundation allocates its funds.
It's a bit complicated so I drew up a little diagram:
1. Mozilla Foundation (not for profit) <- Donations accepted here
├── 2. Mozilla Corporation (for profit, wholly owned by the foundation so all profit goes to the foundation) X- No donations
│ └── 3. Firefox X- No donations
└── 4. Other social causes (??? it's not clear)
At least, that's how I understand it. Honestly I'm super curious about what the benefits of this kind of org structure are, and what objectives they optimize for. This org structure is clearly not an accident...