To disable the fullscreen warning message:

1. Open about:config page in Firefox
2. Search for full-screen-api.warning.timeout
3. Change the value to 0 and save

It's not possible to hide this message from the extension.

Check out this article if you need more help: https://www.howtogeek.com/659105/how-to-disable-firefoxs-fullscreen-warning-message/

Thanks for the review!