Why is my mercury outboard motor not firing?

There are four main things you need to start an engine fuel, air, spark, and compression. You need all four to start your engine whether that be a marine engine or land vehicle. Now, I am assuming it has a carburetor because even my 5 HP 4 stroke Honda has one. And if your does you migh just need to clean it, OR put some quick starting fluid in there. If that doesnt work try just cleaning out the fuel lines and carurator because maybe the fuel isn't getting to where it needs to go. I mean check anything you can all of that stuff I listed above, spark plugs, pistons, cylinders, and even the cam shaft.

Wont hurt to try. Just make sure you don't break anything.