In March 2011 the company, which supplies gas to homes in the greater Belfast area, promised to keep prices the same until at least October of the same year.
But on Tuesday it was announced that customer's bills will rise by 35% from 1 October 2011.
Michael Scott, Acting General Manager of Firmus, explained why the company decided to introduce a price hike.
"Whole sale gas prices have increased by 50% over the last year and 100% over the past two years. We're still offering choice for customers in the greater Belfast area," said Mr Scott.
"If we could've maintained our price freeze, we would have. However, we haven't been able to do so."
John French, Head of Energy at the Consumer Council said the news was the latest in a series of increases in home heating bills.
"This is further bad news for consumers, in a year that has seen other large price increases in electricity, home heating oil, gas and coal - especially given that Northern Ireland has higher energy bills than England, Scotland and Wales," he said.
Figures released by the Consumer Council show that the majority of households in Northern Ireland pay £2,114 a year for oil and electricity - that is more than double the highest energy bill in Great Britain, where households in Cardiff pay around £1,100 annually.
But Firmus's Michael Scott insisited the company is still offering a discount to customers, after rivals Phoenix Natural Gas increased their prices by 39% in May 2011.
"At that time, Firmus promised to freeze prices until the end of September and after that remain at least 7½% cheaper than Phoenix.
"We can increase the level of discount from 7½% to 10% in year one and 5% in year two," added Mr Scott.
South Belfast MP Dr. Alasdair McDonnell said the Firmus price rise, coupled with a recent survey by the Citizens Advice Bureau which shows that one third of elderly people have had to choose between heating and eating, means increased charges must be rationalised.
"Urgent action is required to tackle fuel poverty and customers deserve clear and transparent reasons as to why price hikes tend to occur just before winter and why apparently customers in Belfast in this instance are being singled out and treated so unfairly," he said.
Strangford MLA and Ulster Unionist economy spokesperson, Mike Nesbitt, said the privatisation of the energy market has not provided the expected choice for customers.
"What we are in danger of supporting here is confusion, rather than choice. I understand Firmus are still offering gas more cheaply than their rival," said Mr Nesbitt.
"I also appreciate they are no different from any private sector organisation, out to make a profit.
"But we need to keep people straight about that is going on, and in this case, the plain fact is a 35% rise is a 35% rise, and that is more devastatingly bad news for consumers, domestic and commercial."
