Greetings & welcome Sidney10, we’re glad to have you as part of the community!
You would be better served by asking an attorney. However, I will say that marijuana is, Federally, illegal and considered a Schedule I controlled substance which, by itself, would prohibit you from either purchasing or possessing a firearm.
Form 4473, the one filled out when purchasing a firearm, specifically asks this question (21.f.) Firearms Transaction Record
@Sidney10 Welcome to the community! Great questions. Legalizing Marijuana (where States accept them) is like a two-way street going ONLY in ONE DIRECTION “HEAD ON COLLISION” (damned if you, damned if you don’t) just like any other “TRAP”. Treat this just like you would alcohol. You DON’T DRINK AND DRIVE (especially with a firearm). Medicinal or not. You DON’T TOKE AND DRIVE (especially with a firearm). Now,(1) Working in a hemp product store, check the company policy for firearms IN THE WORK PLACE (doesn’t matter what the work place is, they all have policies). If there’s none, and it’s not a gun free zone, and it’s ok with JUST YOUR BOSS, then be the grey man . (2) Medical MJ License and carrying while in possession FEDERALLY’NO". STATE" " Bad mixture (I would Never chance it). (3) If you don"t have you MJ card with you and have the goodies and carrying outside of work (out in public?) , same as (2). Check with a lawyer for these answers (since the legislators/lawmakers are the ones that bring these things to the table with no clarifications). Too many grey areas that need to be addressed. So, it’s best to JUST enjoy it in the comfort of your own home, or have a designated driver and DON’T be armed if you’re out and about in town partying. It’s more like a “TRAP” for another means to get you, and DISARM you . “GOTCHA ”
It’s all a bit complicated, but states don’t technically “legalize” marijuana, they can’t.
As long as it remains illegal on the federal level, article VI, clause 2, of the Constitution (the Supremacy Clause) states that federal laws pursuant to the Constitution are the law of the land, meaning no state law can make legal what is illegal at the federal level.
If marijuana is ever reclassified (or less likely declassified), the laws regarding firearms & marijuana would likely be very similar, if not identical, to those pertaining to firearms & alcohol. This is a “remains to be seen” hypothetical
As it is today, it is not legal to use marijuana and either purchase or possess a firearm.
To further clarify, Florida is Constitutional Carry and a “shall issue” state. Constitutional Carry means that you can carry without a permit so long as you are within compliance of federal law (the Constitution). The “shall issue” means that so long as a person does not have a situation barring them from possessing a firearm federally (the Constitution), the state “shall issue” a permit. Marijuana is a “situation” that would bar someone from possessing a firearm.
And to make things even muddier, he’s asking if he can carry his gun at a smoke shop that sells cannabis products, while not explicitly saying that he himself is using or under the influence.
Does his working at a shop that dispenses a federally regulated substance preclude his right to bear arms?
AND DRIVE (especially with a firearm). Medicinal or not. You DON’T TOKE 
AND DRIVE (especially with a firearm). Now,(1) Working in a hemp product store, check the company policy for firearms IN THE WORK PLACE (doesn’t matter what the work place is, they all have policies). If there’s none, and it’s not a gun free zone, and it’s ok with 
JUST YOUR BOSS, then be the 
grey man 
. (2) Medical MJ License and carrying while in possession 
FEDERALLY’NO". STATE" 
you, and DISARM 