I tracked down the pilot of the firefighting aircraft in Jason’s video and I am waiting for him to return my call so we can discuss HG/PG compliance with FAR 103.13 including making ourselves more visible in addition to being aware of our surrounding and avoiding powered aircraft. This hazard occurs this same time every year since high altitudes tend to coincide with fires and firefighting operations.
Please check https://skyvector.com/ for a rapid assessment of fire situations and TFRs before flying each day. It’s quick and easy.
AFAIK – firefighting aircraft may not appear on the flight track websites. I located the helicopter that flew across my path crossing Devil Canyon the same day on the ONT flight animation history site but not the BAe 146 that came up behind Jason and passed him.

Jason’s jet in action:

My chopper on the tarmac: