Firmin Didot was born in Paris into a family of printers founded by François Didot, the father of 11 children. Firmin was one of his grandchildren. The family's paper manufactory was located at Essonnes, a town c. 30 km southeast of Paris near Corbeil, which had notable paper factories.
France is indebted to the Didot family for the publication of the Biographie Nationale, and Belgium is also indebted for the establishment of her Royal Press. Relatives of Firmin Didot include François Ambroise Didot (1730–1804); Pierre François Didot (1732–95); Henri Didot (1765–1862); and Pierre Didot (1760–1853).
Essai sur la Typographie by a member of the Didot family was published at Paris in 1852.
Along with Giambattista Bodoni of Italy, Firmin Didot is credited with establishing the use of the "Modern" classification of typefaces. The types that Didot used are characterized by extreme contrast in thick strokes and thin strokes, by the use of hairline serifs and by the vertical stress of the letters. Many fonts today are available based on Firmin Didot's typefaces. These include Linotype Didot and HTF Didot