The plural of fireman is firemen. If you want to use inclusive language, then you can say firefighter and firefighters.
Firefighter's
firefighters'
The possessive form for the plural noun firefighters is firefighters'.
firefighters': The local tinsmith made the firefighters' buckets.
firemen's The firemen's pay was late.
Coaster's is singular possessive. The plural is coasters, the plural possessive is coasters'.
The correct form is "its" for the possessive form in the plural. "Its" is used for both the singular and plural possessive forms, without an apostrophe.
The plural is briefs. The plural possessive is briefs'.
The plural is lads. The plural possessive is lads'.
The plural is trollies. The plural possessive is trollies'.
The plural is stations. The plural possessive is stations'.
The plural is relatives. The plural possessive is relatives'.
Museum is singular. Museum's is singular possessive. Museums is plural. Museums' is plural possessive.
The singular possessive form is heart's; the plural possessive form is hearts'.