Mozilla currently don't provide a dev build for Android, just regular and beta versions https://play.google.com/store/apps/developer?id=Mozilla
The security problem that this "fixes" is not really an issue on Android due to Android's own app sandboxing, so maybe the Android build will allow unsigned extensions? It's not mentioned in the FAQ.
Nightly versions of Firefox are available from https://nightly.mozilla.org/.
not really an issue on Android due to Android's own app sandboxing
A malicious add-on could still steal all your passwords.