Firefighters from Sheringham and North Walsham fire stations have started responding to cardiac arrest calls as part of a new trial.
It is hoped lives could be saved by getting first responders to the scene of cardiac arrests sooner.
But opposition councillors have labelled the trial - to run until autumn - "a sticking plaster" which will just put added pressure on an already stretched fire service.
Paul Seaman, Norfolk Fire and Rescue's head of response, said that while nothing could replace an ambulance crew, getting firefighters to cardiac arrests earlier could mean "the difference between life and death".
Mr Seaman said: "We know that being on the scene as quickly as possible is vital in responding to incidents of cardiac arrest, and it makes sense to see if using our teams based at local fire stations can make a difference."
Liz Withington, a Lib Dem Sheringham town and district councillor, said the trial would strain north Norfolk firefighters and wind up costing their employers, as many worked for the emergency service on an on-call, retained basis.
Ms Withington said: "This is a terrible admission by Norfolk County Council that the current state of our ambulance service and the current response time disaster is not resolvable.
"Perhaps more important is the fact that once a fire unit is attending a cardiac arrest they are not available for fires or road traffic accidents which are both already their main remit."
Ms Withington said crews from Sheringham and North Walsham already went to 21pc of fires in north Norfolk.
Lucy Shires, Lib Dem district and county councillor, said North Walsham's ambulance service was under pressure because its rapid response vehicle was removed last autumn.
She added: "I know our hard working fire teams, will as always, rise to this additional challenge but we have not seen the ambulance response times actually improve. Will our fire teams still have to wait for hours for an ambulance to arrive?"
Councillor Margaret Dewsbury, the county's Conservative cabinet member for communities and partnerships, said fire crews already worked with the East of England Ambulance Service on a daily basis.
She said: "This trial will take that co-operation in a new direction.
"Rapid response is vital when it comes to cardiac arrest, and co-responding may save lives in that crucial, early period."
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here