With newer panels there is no top or bottom.
Most newer main breaker panels have a breaker that moves side-to-side so even those can be mounted with the main breaker at the bottom.

The issue is that breakers cannot be installed where "OFF" is in the up position and "ON" is in the down position. Side-to-side motion is fine.
This applies to the US. I don't think this is a code issue in Canada.

The rules say that when the handles are oriented vertically, ON shall be up.
As pointed out a panel with horizontal breakers can be put in either way.
The instructions will generally also tell you the allowable installation orientations.

Every house in the development I grew up in (as well as everyone else built by the same builder) had the old PUSHMATIC panels installed upside down ... the supposition as it saved a lot of money even back then on the service conductors being three feet shorter. Of course the breakers read "NO" when on.

I totally remember our panel saying "no" when I was a kid!

Thanks, guys.

Sorry, shoulda said I was in the US.

The box I have has the breakers laying horizontally.

I guess I will be OK with it "upside down".

Thanks again.

Was it a Levitt house?