The Firehole River got its name from its early trappers who saw steam rising from geothermal areas near its valley and assumed it was caused by fires. The term "hole" refers to a small valley, thus "Firehole". It has a very interesting and unique ecosystem due to the elevated temperatures of the water caused by hot springs and geysers nearby.
The river winds its way through the western part of Yellowstone National Park and actually bisects Upper Geyser Basin. This image was taken from a bridge in the basin. You can see that the banks of the river in many places are stained white, orange and brown from mineral deposits and bacterial mats.