Upvote:2
My French passport expired 2 weeks ago.
Huge mistake, as you are about to find out.
Contact the Canadian embassy in Mexico City or any of the numerous consulates. They might give you a travel authorization. They have no obligation to do so, but it's Canada, eh.
You DO have a valid Permanent Resident card, yes? Gets much harder without one.
If Canada says the travel authorizations are only for citizens, then your only reasonable* option is the French Embassy in Mexico city, conveniently a 20 minute walk from the Canadians. Also talk to your hotel about extending your stay until your passport arrives.
Come back and tell us what worked (and what didn't work).
* You could take a boat through the Caribbean, stay more than 12 miles from US coasts, and land at St. Pierre et Miquelon. Then it's a ferry to Newfoundland. Your French citizenship gets you onto St. Pierre, your PR card gets you back into Canada. I can't imagine this costing less than staying in Mexico for upwards of a year or two.