It looks like Mozilla doesn’t maintain an official Canadian dictionary, they just rely on this add-on created by a single guy: https://addons.mozilla.org/en-US/firefox/addon/canadian-english-dictionary/

I’d suggest turning that dictionary off and relying on a third-party spelling/grammar app with better Canadian language support. I don’t know if they have a Canada-specific option, but I like LanguageTool: https://addons.mozilla.org/en-US/firefox/addon/languagetool/

There are a lot of options out there, be careful about which ones have a decent privacy policy. Keep in mind, they could easily record anything you type, on any website.