If I used to live in Quebec where I earned my self employment income but now live in Ontario (as at December 31 2018). Which address do I put under self employmemt?

You will need to provide what province you physically lived in on December 31st,2018 in the residence section.

Then Self-employment income address which would be QC can be applied, along with the start and end date (if any), this is entered under the Income tab then Self-Employment, the QC portion will follow the federal entries.