Testfor command not working

It's not working because of "firelordarmor 2". That's not a part of the command and shouldn't be there, delete it.

Also you need quotes around the names of the items. So the fixed command is:

/testfor @p {Inventory:[{Slot:103b,id:"minecraft:skull",tag:{display:{Name:"FireLord's Skull"}}},{Slot:102b,id:"minecraft:leather_chestplate",tag:{display:{Name:"FireLord's Chestplate"}}},{Slot:101b,id:"minecraft:leather_leggings",tag:{display:{Name:"FireLord's Leggings"}}},{Slot:100b,id:"minecraft:leather_boots",tag:{display:{Name:"FireLord's Boots"}}}]}

The Minecraft wiki tells you the syntax for all commands. With it, you could have avoided the first problem.