We have 2 firesticks for a couple years now and have never seen a thermometer icon pop-up and one big screen is on 16 hours a day or so. We have tons of apps installed too including HBO, STARZ Showtime CNN, MSNBC and tons of others.
Firesticks do get hot sometimes. The one on a lot has auto rebooted maybe 3 times, and that was summertime.
You should get an HDMI extender to move the stick farther away from your TV
