When editing web extension source code, reload the extension in Firefox so you can test it as you save your source files. There might be a couple ways to do this.

Firstly, the platform supports a special symlink file: If you put a text file in profile/extensions whose name is the addon id and the content is the absolute path to your addon source, you can reload it.

To reload it, you just need to disable it, flush the cache, and re-enable it (see flushJarCache() and flushStartupCache()). We will add a button for this but ideally web-ext could use the debugging API to send a message to Firefox to reload when necessary.

When reloaded, the add-on will see the shutdown and startup methods called, it won't see the uninstall/install called, but you probably don't want that for reload anyway

This reload trick won't work for XPI paths, only for directory paths.