To find the firing order, you should:
1. Check your engine: Look for an engine ID tag, usually on the side of the engine block. This will tell you the exact engine displacement and code.
2. Consult a repair manual: A repair manual specific to your E250's year and engine will list the firing order. You can find these online (often as PDFs) or at auto parts stores.
3. Use an online resource: Some websites allow you to search for firing orders based on year, make, model, and engine code. However, double-check this information against a reputable source.
Without knowing the engine, it's impossible to give you the correct firing order.