The truck turned up outside Tettenhall Fire Station, in Regis Road, at around 10.40am, before a man and woman both fled the vehicle.
Crew commander Daniel Cartwright said: "We were just doing our duties at the station when a flat bed van parked up in front of the station.
"It was carrying bathroom waste and garden rubbish which was on fire.
Firefighters tweeted this image of the aftermath
"A man and woman jumped out. They had been flagged down by someone nearby who said the back of the truck was on fire.
"I saw it from the window and we reacted from there."
The road was closed for around an hour while they dealt with the fire, which caused minimal damage to the truck.