Judges retired from the bench often do "consulting" - a form of advising by another name. Those still sitting often do give speeches on various legal issues (the CJ of the SCC is a good example), but (unlike the US Subprimes) they rarely engage an issue, except in a very soft and fluffy way. Substantive remarks out of court on hard issues ex cathedra are still very much frowned upon.

Ditto what Go Canucks said. It's unlikely that a judge would be endorsing actively anything. Joining a law firm might, however, be a kind of tacit endorsement of the standing of the firm.