What is the firing order for a 1959 Ford F-350 4.8L engine?

The 1959 Ford F-350 likely didn't have a 4.8L engine. Ford's engine offerings in that year for the F-350 were significantly larger, typically in the 262 cubic inch (4.3L), 292 cubic inch (4.8L) or larger displacements. There was no 4.8L *designated* as such. The 292 was the closest.

Furthermore, firing order information for engines of that era isn't consistently documented in a readily accessible, centralized database. You'll need to consult a 1959 Ford F-350 shop manual or a very detailed parts catalog specifically for the exact engine size (292 cubic inch is most probable) in your truck.

Without knowing the precise engine displacement and potentially the engine code stamped on the engine itself, providing the correct firing order is impossible.