I was scared too, but I verified it was actually the verified FireFox account DMing me, and although I didn't check the store very well (I was very excited, very bad mistake I know), it all seemed legit. I remember before I got gifted the coupon, I thought: There's a Mozilla store? So I searched it online and then verified that it was the same link they sent me (still did it though a search engine just in case, and the coupon worked as intended).

I don't know what else I could've done to verify it, but I'd like to be optimistic, specially since it's such a random event lol (Even if I did end up getting scammed, I only lost like 10 usd lol)

EDIT: FireFox just liked my last post about the purchase, so unless they were hacked, it's legit!