This was a big deal after the Camp and Complex fires because of some viral articles that drew well justified outrage among Californians so they went a step above and just allowed them to expunge their records completely (so that they can get hired everywhere else too, they don’t have to become fire fighters).
There are some stipulations like the sentence they served must be at most 8 years, but they’re not all banned from continuing their work.
Well good, since professional and volunteer firefighters are the most common arsonists. People with violent criminal records should not be in those fields at all.
Your article cites 100 annual firefighter convictions for arson in North America. For your claim to be true, that means fewer than 200 arson convictions annually in all of Canada, US, and Mexico. It says firefighter arson is common, but does not appear to claim that they are the most common offender.