This is Air Tanker 137, a B-737 operated by Coulson Aviation. It’s one of two 737s they have converted to air tankers. They sold the other one to New South Wales in Australia a few months ago.
This aircraft, T-137, made the first ever drop on a fire by a 737 November 22, 2018 during the recent 2018-2019 Australian summer while it was on contract down there before the other 737 was delivered to NSW. It made its first drop in the United States around the first of August this year.
Britt Coulson said the company substituted T-137 for their T-132, an L-382G that was on a Call When Needed Contract, taking the place of it on the list.
Obviously Coulson Instagrammed the photo, but if someone knows the photographer’s name, we will add it here.