Three men, based in Bury, are being investigated for "involvement in the use of unauthorised rest facilities". It is claimed they broke regulations by using sleeping bags on the floor rather than the £400 chairs.
The chairs were installed as part of modernisation programme to replace all beds in the region's 41 fire stations.
The Fire Brigade Union (FBU) said the men were all asleep as a team of inspectors from the fire service carried out a spot check one morning at 0630 BST.
"We have now christened them the furniture police," said Manchester regional secretary Kevin Brown.
Mr Brown said the service launched an investigation into the incident and the men were due to appear before a level three disciplinary hearing on 14 June.
"A level three hearing leaves open the possibility for dismissal - this is how ludicrous this is," said Mr Brown. "Obviously what we are looking for is for common sense to prevail. These people work a 15-hour night shift and they are entitled to take rest periods."
Greater Manchester Fire and Rescue Service said the inquiry concerned "involvement in the use of unauthorised rest facilities". "A full internal investigation into this matter is under way and no further comment can be made at this time," a spokesman said.
The service bought more than 300 of the chairs last year after chiefs decided to remove beds from dormitories across the region.
But firefighters were not allowed to sit or lie on the devices before reading a four-page health and safety manual.