Firefox's build has been uninstallable in Firefox. Attempts to sideload it will give an error saying it's corrupt, and it's missing from Firefox's add-on store.

Currently the only way to add it is to go to about:debugging#/runtime/this-firefox.

This might be an issue with signing, since it was delisted and Firefox only allows signed addons to be installed. However I have failed to install it on the Developer Edition, so it could be something else.