If Wildfire Eternal attacks, and isn't blocked, the player is allowed to cast a sorcery or instant from the hand without paying its mana cost.
If Dread Wanderer is in the graveyard, you can return it to the battlefield if you can cast a sorcery and you have one or fewer cards in your hand.
Assuming you already have one or fewer cards in your hand, and have attacked with Wildfire Eternal that hasn't been blocked, can the player now instant cast Dread Wanderer from the graveyard?
No, you can't do that.
When an activated ability says "activate this ability only any time you could cast a sorcery", that is placing additional restrictions on the normal rules governing when you can activate an ability (normally, when you have priority).
307.5. If a spell, ability, or effect states that a player can do something only “any time he or she could cast a sorcery,” it means only that the player must have priority, it must be during the main phase of his or her turn, and the stack must be empty.
No this doesn't work like that. There are a few reasons:
The Dread Wanderer's ability can only be played at sorcery speed. That's all that "Anytime you could cast a sorcery" means. It means you can only play it on your main phase when you have priority. Also as its an ability that returns the card, it is not actually cast.
Wildfire Eternal allows you to cast an instant or sorcery. As Dread Wanderer is not an instant or sorcery card, the interaction would not work with Wildfires ability. There's also the point that Wildfire states it has to be cast from your hand.
If you happened to have one card left in hand, you could play Dread Wanderer's ability on your second main phase provided you have the manna.
