Firefox recently introduced two new APIs for WebExtensions, called Content Script API and User Script API. The APIs are supposed to replace the current methods of injecting remote code (code not available in the extension when it is submitted to the Firefox Add-ons store). The current methods of injecting remote code (e.g., eval() and creating <script> tag and inserting code into it) will be deprecated and removed eventually (no exact timeline yet, though).

Blog:
https://blog.mozilla.org/addons/2019/03/26/extensions-in-firefox-67/#userscripts
MDN Docs:
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/userScripts
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/contentScripts