I was let go as part of the company downsizing, and a couple months later found out that my old position was filled externally. Is this grounds for a wrongful dismissal?

Your without cause termination as a result of downsizing would be considered a wrongful dismissal and you are owed severance. I would encourage you to visit the severance pay calculator our firm has developed to see what you are owed (www.SeverancePayCalculator.com). I would also encourage you to call us and set up a consultation with one of our employment lawyer who will further discuss your matter with you.