Preference to turn off add-on signature check is now available in Firefox 40. Mozilla will enforce extension signing from Firefox 40 version (scheduled to release on August 11th) onwards, Firefox browser vendor has already reviewed and signed all add-ons in AMO, they’re in the process of reviewing non-AMO add-ons.
Disabling Extension signing in Firefox 40
FYI: developer of add-ons can disable signing requirement by changing about:config preference xpinstall.signatures.required value to ‘false’ in version 40. This preference will be removed in Firefox 41 (expect this to exist in Nightly and dev. editions).
Developers can still test their unsigned add-ons in developer and Nightly versions as well as unbranded builds of release and beta.
There will be a transition period of two release cycles (12 weeks total) during which unsigned extensions will only generate a warning in Firefox.
After the transition period, it will not be possible to install unsigned extensions in Release or Beta versions of Firefox. There won’t be any preferences or command line options to disable this.