1999 Mercury Sable Hard starting

Car has been sitting for about 2 months due to a blown brake wheel cly. I replace it and rear shoes. I had to charge the battery a couple times to get the car started. It was backfiring when cranking. I replae the plug wires but didnt do the plugs. The backfiring just about went away, but still had to crank for a few minutes before it would start. The car would start and run great before she let it set. When she gets it started now it does run great. She did have the battery tested and it had a bad cell, and she had it replaced.
I am thinking it might be a fuel problem. Maybe fuel filter? Not sure if it has a inline filter or if its in the gas tank
Any advice?