In Firefox 74 you will need to use extensions.experiments.enabled
instead of extensions.legacy.enabled. This should land soon in bug 1524327.
What is this?
This preference is used to enable to features for developers in Nightly,
Developer Edition and unbranded builds. It allows testing of
experimental APIs without signing an extension, as well as testing
privileged APIs when loading the extension through about:debugging.
History and reason
When we were making the change from the legacy xul/xpcom extensions to
webextensions, we had a build flag and preference to continue allowing
the use of legacy extensions. The ability to load legacy extensions has
long been removed from Firefox, but the flag was also used for these
features and has remained to support them.
The name change will clarify what the preference does and allow us to
remove some remaining bits of code that supported legacy extensions.
