How can I disable signature checking for Firefox add-ons?

Since version 42, Firefox, by default, refuses to install unsigned add-ons. How do I disable this verification?

Why do I have to keep signing in to Firefox?

Your 'logged-in' status is stored in special cookies. If the cookie is removed, you are logged out. You may be in Private Browsing mode or told Firefox not to remember. Type about:preferences#privacy<enter> in the address bar.

It is only possible to disable addons verification in Nightly and Developer channel. In other words it is not possible in Beta versions and standard releases.

Go to about:config (enter it into address bar)
Set xpinstall.signatures.required to false.

More at https://wiki.mozilla.org/Addons/Extension_Signing