The Firefox DevTools don’t have a panel that shows all files at one place the way the Chrome DevTools do it. Though there are different specialized panels showing you the same info. E.g. the Style Editor shows you all CSS sources. The Network Monitor shows all requested files with focus on network request information.
The Debugger is meant to debug JavaScript. So, it only displays JavaScript sources.
The Storage panel is enabled by default in newer versions of the Firefox DevTools. To enable the Storage panel in older versions of Firefox you need to switch to the settings and click the checkbox besides Storage:
Though the purpose of the Storage panel is to display storages like cookies, the IndexedDB, the local and session storage. It does not display files.