In the professional literature, there is no information about the sieve and morphological composition OFMSW (<80 mm) subjected to biological processing in MBT installations during the heating season. Knowledge about the quality of this fraction is important because it significantly affects the course of the biostabilization process. The paper presents the morphological composition, sieve composition, humidity, loss on ignition and organic carbon content of <80 mm fraction separated from municipal waste delivered to 21 MBT installations in Poland, in winter. The results show that about 1/3 of the OFMSW (34.0 ± 10.6%) mass is a fine fraction (<20 mm), mainly furnace waste. The organics share was on average 39.3 ± 10.4%. The fraction “paper and cardboard” was also present in the bulk of OFMSW, an average of 10.5 ± 4.3%. The shares of other components did not exceed 5%. The large number of batteries in OFMSM was surprising, despite their widespread selective collection. The high content of fine fraction (ashes) and inert components in OFMSW during the heating season results in low hydration and low losses of waste incineration, which may even determine their inability to be processed biologically.