Possession of a stolen firearm is a serious charge in Florida, as the law basically combines the concepts of illegal possession of a firearm with possession of stolen property, both of which can be felony charges. If a person is found to possess a stolen firearm, he or she will be charged with a third-degree felony at a minimum, and perhaps a second or even first-degree felony with the presence of certain facts that work against the defendant.
Third Degree Felony - A third degree felony is punishable by up to five years in prison, five years probation, and a $5,000 fine.
Second Degree Felony - A second-degree felony is punishable by up to fifteen years in prison, fifteen years probation, and a $10,000 fine.
First Degree Felony - A first-degree felony is punishable by up to thirty years in prison, thirty years probation, and a $10,000 fine.
Because of the marijuana possession, you would likely be looking at a second degree felony.
My suggestion is to get a criminal law attorney to help you get the marijuana charge dropped and keep the matter a third degree felony, and see what kind of deal you can cut with the prosecutor.