Firelord is way weaker then either Surfer or Quasar, only Guardians he actually defeated were pretty weak (he didn't defeat Starhawk or anything), they would have defeated him if Crimson Bands hadn't backfired, and even when he all-out slapped Firelord, he was only thrown back, not knocked out.
Like shown before, Keeper obviously was able to do jack shit to him when he had fed. And when Keeper was stopping G from eating planets, he did not face him directly, he "acted in clandestine manner until Galactus was weak enough so he could actually defeat him". This was stated in the comic.
Keeper had to face the most weak Galactus in the history of continuity to defeat him. Thanos could face being more powerful then Galactus and actually survive his major attacks. Not to mention Galactus himself. He also defeated Kosmos, who is about equal to Beyonder...he faced guys like Tyrant or Odin.
Keeper is under Thanos.