The only direct way currently is to pay for associated products like Firefox VPN, assuming it's available in your region. That money will go directly to Mozilla

The only direct way currently is to pay for associated products like Firefox VPN, assuming it's available in your region. That money will go directly to Mozilla Corporation, which develops Firefox. Donations go to Mozilla Foundation, which does advocacy and various world-improving things (IMO). Mozilla Corporation cannot legally accept direct donations IIUC. I think it'd be a money laundering loophole or something? Servo was never going to grow into a complete browser, and much of it was absorbed into Gecko (the Firefox codebase). But I agree, it would be great to have the Servo idea pushed further forward.