  We report 250 GHz (1.2 mm) observations of a sample of 20 QSOs at redshifts
5.8<z<6.5 from the the Canada-France High-z Quasar Survey (CFHQS), using the
Max-Planck Millimeter Bolometer (MAMBO) array at the IRAM 30-metre telescope. A
rms sensitivity <~ 0.6 mJy was achieved for 65% of the sample, and <~ 1.0 mJy
for 90%. Only one QSO, CFHQS J142952+544717, was robustly detected with
S_250GHz = 3.46 +/-0.52 mJy. This indicates that one of the most powerful known
starbursts at z~6 is associated with this radio loud QSO. On average, the other
CFHQS QSOs, which have a mean optical magnitude fainter than previously studied
SDSS samples of z~6 QSOs, have a mean 1.2 mm flux density <S_250GHz> = 0.41
+/-0.14 mJy; such a 2.9-sigma average detection is hardly meaningful. It would
correspond to <L_FIR> ~ 0.94+/-0.32 10^12 Lo, and an average star formation
rate of a few 100's Mo/yr, depending on the IMF and a possible AGN contribution
to <L_FIR>. This is consistent with previous findings of Wang et al. (2011) on
the far-infrared emission of z~6 QSOs and extends them toward optically fainter
sources.
