Hes not hes very durable more then FFF and soem other not taking damage from thors hammer he also can shto otu mystical flame and he probabyl could beat FFF cause hes stronger then thor and thor can already lift beyond 100tons and we all say when hulk fought FFF he beat him hes about 6'7
"Fin fang foom will bring down fanfir the wyvern down. It may not be easy but having two more limbs than his opponent will help. Or is fanfir a normal dragon in marvel?"
Fafnir is a massive green scaled dragon - very thick and stubby of body by comparison to most dragons.
He could breath potent fire, his hide was extremely durable - so that he took little damage from Thors hammer blows (so more durable than the Hulk). His strength was supposidly greater than Thors and he had some illusion casting powers. All things considered, Thor was going to have a hard time beating him without help (killing him by driving a magic spear in to his heart with his hammer).
Fing fang Foom at the height of his powers is larger than Fafnir and appears to have greater strength. His durability may be slightly less than Fafnirs but he can breath a very nasty acid mist, is quite dexterous, can fly, shapeshift (in to human forms - or shrink himself - not sure if he can other things), has some telepathy 9not sure if it is just communication or if it includes mental attacks - but likly to include some mesmerism) and finally he is a very skilled hand to hand fighter.
Based on this, I would still give it to FFF when at the height of his powers.