reversible encryption

All encryption is reversible, otherwise it wouldn't be encryption, it would be a hash. If you don't use a password, it's easy to reverse the encryption. If you do use a password, the maximum security with a brute force attack is 112 bits, which is pretty weak.

I recommend using a different password management service (which also handles credit card info), any password manager will be fine. I personally use Bitwarden, which uses 256 bits of encryption. That's pretty standard across password managers, so you're better of focusing on making a secure password.

That said, if you're only worried about credit card info and not storing passwords in Firefox, you're probably fine. Credit cards have a ton of protection, so if someone steals your card info, call your bank to dispute the fraudulent transactions and get a new card, it doesn't cost anything and has little hassle. Debit cards are another story, so I recommend just not using debit cards at all online.