In that case you most likely have grounds for legal action if they fire you.

My understanding is companies are allowed to fire disabled employees if their disability prevents them from using necessary safety equipment or doing any necessary aspect of their job. The only thing they have to offer is reasonable accommodation to work around the disability. From the sounds of it reasonable accommodation is available in the form of a different but equally effective mask. They are just refusing to allow you to use it.

These types of cases are generally handled through your province's human rights tribunal. These tribunals are generally designed specifically to ensure that you can get access to justice without hiring a lawyer so you may want to take a look at your province's website to see how to file a complaint. Alternatively you can contact an employment lawyer in your area. Some firms will offer free consultations to help you figure out if you have a case.

Note that employers may anticipate a complaint and try to fabricate some bullshit case against you and claim that's why you were fired so if you want to go this route try to keep your nose clean.