Enable Developer Options on the FireStick
Go to Settings
Select My Fire TV
Select About
Highlight the Fire TV device name
Press the Select button 7 times
Go back to My Fire TV
Open Developer Options
Turn on Apps from Unknown Sources
Turn on ADB Debugging if needed
Install the Downloader app from the Amazon Appstore
Open Downloader
Allow the requested permissions
Enter the Stremio APK download URL
Download the Stremio APK file
Open the downloaded APK
Select Install
Wait for the installation to finish
Open Stremio from Your Apps & Channels
Sign in or create an account
If needed, use a file manager app to install the APK instead of Downloader
If the app does not appear, check that unknown sources are enabled
If installation fails, download the latest Fire TV-compatible APK version