nsIBinaryOutputStream
ew void setoutputstream(in nsioutputstream aoutputstream); void write8(in pruint8 abyte); void write16(in pruint16 a16); void write32(in pruint32 a32); void write64(in pruint64 a64); void writeboolean(in prbool aboolean); void writebytearray([array, size_is(alength)] in pruint8 abytes, in pruint32 alength); void writebytes(alength)] in string astring, in pruint32 alength); void writedouble(in double adouble); void writefloat(in float afloat); void writestringz(in string astring); void writeutf8z(in wstring astring); void writewstringz(in wstring astring); methods setoutputstream() sets the stream to which output is directed.
... writedouble() writes a double precision floating point number to the stream.
... void writedouble( in double adouble ); parameters adouble the double to write to the stream.

Writing to Files - Archive of obsolete content
writedouble will write a floating point value in eight bytes.