Audible Volume Control is an extension that was created because I personally was tired of not being able to control the volume when using the Audible web-player.
At the time of writing this, Audible has just announced that they are discontinuing their native Audible app for Windows (it will probably be replaced with the Android version in Windows 11). Soon, the only option will be the web-player.
Should work for all domains.
NOTE: Don't forget to enable the addon for the specific Audible page you visit, otherwise you have to allow it to run every time.
WARNING: This addon is no longer compatible with older versions of Firefox due to migrating to manifest version 3. Sorry for any inconvenience.