Can I sue a professional's firm instead of the individual who advised me in Ontario?

Usually yes, and it's often the more practical route. A firm - whether a partnership, professional corporation, or employer - is generally vicariously liable for the negligence of its partners, associates, or employees when they act within the scope of their normal work for the firm. That means you can typically name the firm as a defendant alongside, or instead of, the individual professional who actually advised you, and recover from the firm's assets and insurance rather than relying on one person's personal resources.

This matters practically because firms usually carry professional liability insurance and have more assets to satisfy a judgment than an individual employee might. It can get more complicated if the professional was acting outside their normal role, engaged in personal conduct unrelated to the firm's business, or if the firm structure limits liability in a particular way, so the exact relationship between the individual and the firm matters. A lawyer reviewing your claim would typically want to see the engagement letter or retainer to confirm who you actually contracted with and who employed the professional at the relevant time.