Oil and gas are the obvious ones, but I wanted to get more creative
Tire fires, where tires are stored, dumped, or processed, exist in two forms: as fast-burning events, leading to almost immediate loss of control, and as slow-burning pyrolysis which can continue for over a decade.
Tire fires can last a decade or more and are very difficult to extinguish.
A few examples of fire that have been burning for year involving gas. A simple fire that hits some gas like the eternal flame in Turkey, it's a burning of leaking methane gas from deep under ground but it had to be lit at some point. So just a normal fire or forest fire ?
Another is a drilling for oil accident that collapsed because it was on a huge cavern of methane, this was attempted to be burned off in Turkmenistan, the fire has now been burning for 40 years.