Brian,
I echo the other comments, not too much conversation is done between firefighters. Usually the commander tells a crew what to do, and they go do it - not necessarily over the radio.
In Vaughan, mostly everyone carries a radio - but it's rarely used by anyone but the captain. Everyone else listens.
But it is true that some departments switch to a simplex/talkaround channel when they get on scene - I know one department up here (in Muskoka) that does it religiously. Another department is looking at doing it because some areas aren't hitting their repeater and it's just easier to use a simplex channel (using the same main frequencies, just not going through the repeater). However, others have issues with not having everything recorded by dispatch (if applicable).
Kevin P.