PCSO Hannah Moseley, who has previously worked in Dudley, told the suspect: "You're not bringing a gun on my patch."
The drama unfolded when crews were called to reports of smoke coming from an electricity supply box. The mains cable was linked to a house where electricity was being illegally diverted to power a cannabis factory with 300 plants.
Police say a man walked past the firefighters as they made the scene safe and pointed a handgun at two of the crew members.
PCSO Moseley who was on routine patrol moved in to detail the man until colleagues arrived to arrest him.
Hannah, who has been a PCSO with the force for 10 years, said: "I went to give our fire colleagues a chuck with the cannabis farm when one of them pointed out a man they said had pointed a gun at them.
"They felt it was an imitation − but some of these fake guns look like the real thing and can be very scary for people.
"I knew a response car would be a few minutes arriving and didn’t want the bloke to get away…so I went after him, grabbed his arm and told him you’re not bringing a gun on my patch.
"He could see I was really angry − I think he was a bit shocked − and I kept hold of him until back-up arrived to arrest him.
"I can’t believe anyone would point a gun, real or otherwise, at fire or ambulance crews: these are people who are here to protect and save lives."
The drama unfolded in Gregory Avenue, Weoley Castle, Birmingham, at around 7pm yesterday.
Lee Baker, Station Commander at Northfield fire station, said: “We’re very grateful to PCSO Moseley and colleagues for their support at this incident and prompt arrest of a suspect.
“Eight of our firefighters responded to a 999 call reporting a fire in Gregory Avenue. When they searched the house it soon became clear they were dealing with a suspected cannabis ‘factory’.
“These places can be complex and dangerous for our crews, so for them then also to be faced with a man with a fake gun is beyond words.
“Any threat to, or attack upon, our crews is one too many and will not be tolerated. As in this case, we will work closely with West Midlands Police to bring culprits to justice.”
Birmingham Police Inspector Lee Trinder said: "Hannah’s hugely passionate about her job and the communities she serves − I’m not surprised by her actions, it’s in her nature to do the right thing, but she still deserves lots of credit for her bravery.
"She’s a real credit to the team…not just last night but on every shift."
Police are investigating the cannabis factory.
A man aged 20 has been arrested on suspicion of possession of an imitation firearm with intent to cause fear of violence.