Around 300 firefighters, many with their families and wearing their tunics despite threats of disciplinary action, held a demonstration in Hull on Saturday.
Humberside Fire and Rescue wants to reduce the number of crew on engines attending calls from five to four. In all 170 firefighters and managers' jobs could be phased out.
The FBU says the fifth officer has a safety critical role and the move will cause delays with firefighters having to wait for back-up before going into burning buildings.
One firefighter from Calvert Lane fire station said protocols meant that if there were four crew they could only put people into buildings for "snatch" rescues, if they knew exactly where victims were, but "if they are upstairs burning in their beds an officer cannot commit them".
Another from Bridlington said: "It's not a case of militant action. It is genuinely taking action for members of the public. Riding fours is a threat to the public as well as our safety."
A decision will be made at a meeting of Humberside Fire Authority, made up of councillors from four Humber authorities, in January.
Humberside Fire and Rescue insists it will not compromise the safety of crews or the public, and that the service can be run as efficiently using less resources.
The officer in charge of the Integrated Risk Management Plan Daryl Oprey, a firefighter for 25 years, told the Yorkshire Post last week that the "public won't see any difference", adding: "Every fire and rescue service has safe systems of work which enable professional managers and firefighters to dynamically assess the risk and implement a safe system of work, whether five turn up first or four turn up first."
Consultation is continuing until December 31.
Meanwhile in South Yorkshire the FBU has been warned to expect a series of measures including fire station closures, reduced staff levels and the loss of some fire engines.