With the release of Firefox 48, extension signing can no longer be disabled in the release and beta channel builds by using a preference. As outlined when extension signing was announced, we are publishing specialized builds that support this preference so developers can continue to test against the code that beta and release builds are generated from. These builds do not use Firefox branding, do not update automatically, and are available for the en-US locale only.
You can find links to the latest unbranded beta and release builds on the Extension Signing page as they come out. Additional information on Extension Signing, including a Frequently Asked Questions section, is also available on this page.