It seems like Mozilla distributes a special version that allows this. https://wiki.mozilla.org/Add-ons/Extension_Signing https://wiki.mozilla.org/Add-ons/Extens

It seems like Mozilla distributes a special version that allows this. https://wiki.mozilla.org/Add-ons/Extension_Signing https://wiki.mozilla.org/Add-ons/Extension_Signing > The Nightly and Developer Edition versions of Firefox have a preference to disable signature enforcement. There are also be special unbranded versions of Release and Beta that have this preference, so that add-on developers can work on their add-ons without having to sign every build. To disable signature checks, you will need to set the xpinstall.signatures.required preference to "false". Otherwise I guess there’s also IceWeasel if you’re on Linux?