In FireFox, upon a Repl loading, it will gain focus and the browser will jump to the Repl location. I have tested this in FireFox 67.0. You can access a page which will demonstrate the behavior here:
Simply open the page in FireFox, wait for the Repl to load and it will jump to the bottom of the page.
It appears that it is gaining focus upon load. It does not happen in Safari or Chrome.