Browser extensions
Developing for Firefox
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Your_first_WebExtension
In Firefox: Open the about:debugging page, click "This Firefox" (in newer versions of Firefox), click "Load Temporary Add-on", then select any file in your extension's directory. The extension will now be installed, and will stay until you restart Firefox. Alternatively, you can run the extension from the command line using the web-ext tool.
Porting from Chrome
https://blog.chosunghyun.com/porting-a-chrome-extension-to-firefox-add-on/