How Does a “First-Seen Safe” RBF Proposal Attempt to Address the 0-Conf Security Issue?

A "first-seen safe" RBF (FSS-RBF) proposal aimed to allow fee bumping only if the replacement transaction maintained the exact same outputs as the original. This meant the sender could increase the fee but could not change the recipient or the amount, preventing a malicious double-spend to a different address.

The intent was to allow users to speed up stuck transactions while preserving a degree of safety for merchants accepting 0-conf payments. However, FSS-RBF was not widely adopted, as it was complex and offered a false sense of security.