The gas company said prices with go up for its 85,000 customers in Northern Ireland.
They described the move as “regrettable” but that the increase reflected the soar in their own costs.
Michael Scott, managing director of Firmus Energy, said: “We work hard to keep our tariffs as low as possible, to offer price stability for customers and give them confidence in natural gas.
“The decision to raise our tariffs is regrettable and never easy, however in this instance the cost of bringing natural gas to our customers has increased and our price rise has been unavoidable.
“Our pricing team has engaged extensively with the Utility Regulator, the Department for the Economy and the Consumer Council for Northern Ireland in relation to our regulated tariff in the Ten Towns licensed area.
“We keep our tariffs under continual review to ensure we are bringing the best possible prices to our customers.”
Most of its customers are in the Belfast area and they will see an even larger increase of 8.8 percent from April 16.
Its remaining customers, in the Ten Towns area, face the 6.8 percent rise from April 5.
This covers Derry/Londonderry, Ballymena, Armagh and Newry, among other places.
Kevin Shiels, the Utility Regulator’s director of retail and consumer protection, said: “We understand price rises are not welcome.
“Following these increases, we would encourage consumers to shop around to make sure they are getting the best deal for their gas supply.”
Last week, another supplier, SSE Airtricity, announced it was increasing prices by 7.7% for its 150,000 customers.