North Yorkshire Fire Service has begun a consultation on reducing the number of full-time staff, using more part-time crews and smaller vehicles.
The planned changes, which would affect six of the county's 39 fire stations, follow a review and if adopted would save around £1.5m a year.
The Fire Brigades Union (FBU) said about 42 full-time posts could go.
The fire service said it hoped to reduce staffing levels without the need for compulsory redundancies.
The stations where changes are proposed are Harrogate, Scarborough, Malton, Northallerton, Ripon and Tadcaster.
The options included in the consultation include replacing fire engines with smaller vehicles crewed by fewer staff or replacing day-crewed engines with a mix of full and part-time or retained staff.
The number of full-time posts being cut would depend on which options are chosen, the service said.
However, Steve Howley of the Fire Brigades Union said he had "serious concerns" for the safety of both firefighters and the public.
He said: "The reductions to the service being proposed are massive - 42 posts is around a quarter of what we have now."
Chief Fire Officer Nigel Hutchinson said the changes were in response to a 34% reduction in call outs over the last ten years and about managing resources better.
He said: "Logic would say if the risk has reduced we need to make some changes to how we resource the service.
"There will be fewer full-time jobs but I am hopeful, if these proposals are accepted, we should be able to implement them without the need for compulsory redundancies."
The public consultation on the plans runs until October with a final decision due in December.