Dec 30 (Reuters) - Canada"s main stock index opened higher on Thursday on the back of gains in energy and mining stocks, while easing worries over the impact of the Omicron variant also boosted sentiment. At 9:33 a.m. ET (14:33 GMT), the Toronto Stock Exchange"s S&P/TSX composite index (.GSPTSE) was up 54.58 points, or 0.26%, at 21,399.23.