Brake fluid got on painted firewall, what to clean with?

Sorry to hear of this .

One way to avoid brake fluid's damaging effects is to swap to DOT 5 silicone fluid , as it doesn't damage paint , and it's superior to 'conventional' DOT 3 & DOT 4 fluids .

As far as the existing damage :
The advice given thusfar is good , and about the only thing you can do .

Best of luck ! Keep us posted .