How to load the extension in development mode

For Chrome / Chromium / Edge / Kiwi / Opera

Load the unpacked extension via manifest.json from the extension/ folder

For Firefox

Temporarily, copy extension/manifest-firefox.json to extension/manifest.json
Load the unpacked extension via manifest.json from the extension/ folder
Once you are done, before committing any changes, revert extension/manifest.json to its original state

For Android Firefox

Connect the Android device to desktop via USB
Enable USB debugger in the Android device and Android Firefox
Go to about:debugging in the desktop Firefox
$ npm install --global web-ext
$ web-ext run --target=firefox-android # This would list the available devices
$ web-ext run --target=firefox-android --android-device=<device-id>
For example: $ web-ext run --target=firefox-android --android-device=A1B2C3D4E5F