As part of the Extension Signing initiative, signing enforcement will be enabled by default on Firefox 46 for Android. All Firefox for Android extensions on addons.mozilla.org have been signed, and developers who self-host add-ons for Android should ensure those add-ons are signed prior to April 12th, 2016.

Extension signing enforcement can be disabled using the preference outlined in the FAQ, and this preference will be removed in a future release.

More information on Extension Signing is available on the Mozilla Wiki, and information on the change to Firefox for Android can be found on Bugzilla under Bug 1244329.

https://blog.mozilla.org/addons/2016/03/17/add-on-signing-enforcement-in-firefox-46-for-android/