I lived in Quebec from January to August 2024. Started working for an ON-based employer in May 2024 and moved to ON in August. Should I file my taxes together?

When you are preparing your tax return, you can only choose one province. Usually, it's the one you lived in on Dec 31st of the year, so in your case that would be Ontario. 

But even if you weren't a resident of Québec on Dec. 31st, you might still have to file a Québec return (as well as your Federal & Ontario returns). I would recommend contacting Revenu Québec to see if you need to.

To call them from Outside Quebec (Elsewhere in Canada or the United States): 1 800 267-6299 (toll-free)

Outside Canada and the United States: 1 800 267-6299 (long-distance charges may apply)

This Revenu Québec page has more ways to contact them: General Information – Individuals