You can disable this in about:config. Go ahead and try it in Nightly if you don't believe me (Set xpinstall.signatures.required to false)Read the article
The Beta and Release versions of Firefox based on 42 and above (Beta 42 will be released at the same time as Firefox 41) will remove the preference that allows unsigned extensions to be installed, and will disable and/or prevent the installation of unsigned extensions.And don't tell me to use Aurora or Nightly, I like using esr so I have both stability, speed (nowadays firefox is slower in every new version due to their new fancy graphics) and I avoid the new Mozilla features as long as possible. Plus in the past I had been exposed to critical bugs with Nightly and Aurora that made it unusable.
This is a good thing.They forgot to say that it will be extremely limited. Take a look at how limited the chromium extensions are.
I have no idea what the fuck this even means.That the old and flexible API will not be available anymore and any addons using it will stop working.