Florence’s decree that no one outside the Catholic Church is saved is absolutely true in the sense that those who lack any and all connection to the Church are damned. But it is possible to have an invisible link to the Church. Being in the Church does not require full, formal communion.
This was the understanding of St. Thomas Aquinas, who spoke of being in the Church in voto (in desire) rather than in re (in reality), and of the Council of Trent, which taught that we can be justified and consequently saved by water baptism or a desire for it.
In the last few centuries has come a refinement of the Church’s understanding of what constitutes the votum (desire) needed for in voto membership. An implicit desire is sufficient. A person who seeks and tries to conform himself to the truth has an implicit desire or votum for Christ and for the Catholic faith because, by seeking to conform himself to the truth, he is seeking to conform himself to Christ (who calls himself “the way, the truth, and the life”) and his Church, even if he doesn’t know it.
Florence’s statement concerning the inefficacy outside the Church of almsgiving and martyrdom is thus to be understood to refer to those who do these deeds in an external fashion that lacks the votum needed for in voto membership.
Imagine a Jehovah’s Witness who ostensibly sheds his blood for Christ. His martyrdom would be ineffective for salvation unless he had the required votum and thus the supernatural love needed to make martyrdom effective for salvation. As Paul says, “If I give away all I have, and if I deliver my body to be burned, but have not love, I gain nothing” (1 Cor 13:3).