In order to avoid updating the Firefox extension every time we want to add a new game title to the list of available games, it is better to host the game_list.js file online and not locally. [Another option would be to create an API online.]
We can do that either on an online hosting service or more preferably on Github's raw content viewer (e.g., https://raw.githubusercontent.com/KAUTH/Retro-Gamez/master/retro-gamez/popup/game_list.js?).
In both cases, it is important to include the CORS header in the host server's response to the browser.
In Github, it supposedly returns a CORS header automatically (needs to be checked).
On an Apache-like server we need to add the following code in the .htaccess file:
Header set Access-Control-Allow-Origin "*"