New purpose built fire fighting aircraft are available. These are designed for the conditions referred to by SNS3Guppy. However they come at a price. As a result some operators prefer to use cheaper older aircraft that have been modified for fire fighting. Naturally these conversions are less capable and less suitable for the task.
The better fire fighting operations, such as Ontario, have new or recent purpose built aircraft such as the CL-415. Others such as Saskatchewan are upgrading their earlier purpose built CL-215s to turbo-prop engines.
Here is a recent flight test article:
Superscooper: Bombardier 415 flight test