FIR - Far InfraRed, the long-wavelength end of the infrared spectrum. As is typical of infrared bands, definitions vary.
When Arthur Schawlow and Charles Townes proposed extending the maser principle to frequencies well above the 22 gigahertz (GHz) of the ammonia microwave maser reached in 1954, they targeted frequencies three orders of magnitude higher, near 300 terahertz (THz), corresponding to one micrometer (1 µm) in the near-infrared. Their choice reflected the technological reality of the time, the long wavelength end of the infrared was a terra incognito of the electromagnetic spectrum, little explored because few sources and detectors were available. One reason that wavelengths longer than about 15 µm came to be called "far-infrared" in the early days of lasers probably was that that part of the spectrum seemed far out of reach.
Technology has come a long way since then, but the far-infrared remains beyond the well-developed parts of the infrared; although, as with other parts of the infrared, the specified wavelengths differ among definitions.
Wikipedia's Infrared article lists multiple definitions of the far-infrared. The first is 15 to 1000 µm, putting it thermal infrared where blackbody emission peaks around room temperature, longer than the mid-infrared and some definitions of the long-wavelength IR. It also cites the International Standardization Organization's definition of 50 to 1000 µm, a definition also used in the McGraw-Hill Dictionary of Scientific and Technical Terms. The infrared article notes that astronomers have their own definition, with 25 to 40 µm the short end and 200 to 350 µm the long end.
Oddly, the bands specified for "far-infrared lasers" differ. Wikipedia says their wavelengths range from 30 to 1000 µm, close to the 40 to 1000 µm I used in The Laser Guidebook. The McGraw-Hill Dictionary lists a far more limited range, from "well above 100 µm" to 500 µm.
A couple decades ago, such inconsistent definitions didn't matter much, because wavelengths longer than 30 µm were a sparsely inhabited part of the spectrum, largely absorbed by air, hard to detect, and even harder to use. Now new technology is upscaling the unfashionable far-infrared neighborhood and redefining it as the terahertz band, which Wikipedia defines as from 100 to 1000 µm, or 300 GHz to 3 THz.
Plot of atmospheric opacity shows the strength of atmospheric absorption in the far-infrared. (Wikipedia art, modified)