When aircraft working wildfires such as airtankers, helicopters, and lead planes, etc. are either finished for the season or are between contracts (see for example my October 16th post on Coulson T-132 having maintenance before going to Australia), major maintenance takes place. To the best of my knowledge, the time table (usually hours flown) and what types of maintenance tasks take place at different times is set forth in the United States by FAA requirements and perhaps also stated in the contracts, some items will be specific to the aircraft model. While I do not know what happens during what I am referring to as major aircraft maintenance, I know that there is a pretty comprehensive list covering all the systems on an aircraft that have to inspected, replaced, etc. during major maintenance.
Some aircraft maintenance happens more often, what I will call for these purposes routine maintenance. I do not know the nature of routine maintenance of aircraft working wildfires, but from what I know of small aircraft, minor maintenance includes but not limited to oil, filters are performed more often and in the world of aircraft working wildfires may be done at night but I am not quite certain. And of course, if an aircraft as an issue that arose requiring maintenance, that aircraft will be pulled off the line until the issue is addressed by the maintenance crews.
I must acknowledge that I am not an expert or otherwise in the know about the specifics of aircraft maintenance That being said I want to speak to the importance of the maintenance of all aircraft including aircraft used to fight wildfires to keep the aircraft safe/ During the off season, so to speak, the contractors supplying aircraft for working wildfires are not sitting around twiddling their thumbs, they are busy performing maintenance of their aircraft.
The Facebook post that I am embedding below is from AeroFlite Aerial Firefighting who maintains a fleet of CL-415 amphibious aircraft, and Avro RJ85 AT and Dash 400 AT aircraft. You will see some photos of their winter maintenance of their CL-415 aircraft receiving maintenance.