If an opponent plays Abandon after I play a spell with Fizz, what happens?

The spell is recycled (put on the bottom of its owner's deck), not returned to hand.

Here's why: Fizz's "Recycle that spell after you play it" and Abandon's "Return it to its owner's hand instead" are both replacement effects that modify what happens when the spell leaves the chain. Per Rule 372, the spell's owner chooses the order they apply. But either path leads to recycling:

• Fizz first, then Abandon: The spell would be recycled instead of trashed. Abandon's effect then has nothing to replace — the spell is already going to deck.
• Abandon first, then Fizz: The spell would go to hand instead of trash. But going to hand still counts as "leaving the chain," so Fizz's effect kicks in and replaces that destination with recycling instead.

Both orders produce the same result — the spell ends up recycled. (FAQ #11158)