Mozilla has allowed extension signing not to be enforced on the Nightly and Developer Edition (Aurora) builds for add-on developers. However, it is enabled by default though when you install or update to these builds. To turn off the extension signing follow these steps:
- In a new tab type: about:config in Firefox’s address bar and hit enter.
- Confirm you will be careful if a warning message is displayed.
- Search for xpinstall.signatures.required.
- Double-click on the preference name so that its value is set to false.
- Restart the browser to reactivate any disabled extensions
The release version of Firefox 41 (September 2015) will still have this option (or something similar) so users can still install/use unsigned/unverified extensions. However, starting with the release version of Firefox 42 (November 2015) unsigned/unverified extensions can not be installed/used.
Note: This does not apply to Firefox ESR until version 45 (March 2016). Also, this does not apply to Pale Moon or any other unbranded releases of Firefox.