Dec 31 (Reuters) - Canada"s main stock index opened lower in thin trading on Friday weighed by energy stocks, but was on track to mark its best year since 2009 supported by massive stimulus, vaccine rollouts, and hopes of global economic recovery. At 9:31 a.m. ET (14:31 GMT), the Toronto Stock Exchange"s S&P/TSX composite index (.GSPTSE) was down 37.78 points, or 0.18%, at 21,256.86.