The short answer is yes, but the CWFL holder is going to have more hurdles to overcome than they would if they were not in violation of the law. Florida Statute §776.012(2) allows a person to use deadly force to protect themselves or others as long as it was reasonable to believe that the use of deadly force was necessary to prevent imminent death or great bodily harm or to prevent the imminent commission of a forcible felony. Under this statute a person who is justified in their use of deadly force has no duty to retreat before employing deadly force so long as they are not engaged in criminal activity or in a place they do not have the right to be.
A CWFL holder who is in a prohibited place is committing a second degree misdemeanor and therefore would be involved in criminal activity. Fla. Stat. §790.06(12)(a)(15)(d). Therefore, if the CWFL holder is in violation of the law, i.e. in a bar, the CWFL holder would be required to retreat before using deadly force to stop the mass shooter. There are two exceptions to this requirement however. The law would only require that CWFL holder to retreat if they had the opportunity to do so and if retreating would not expose them to death or great bodily harm.