Some modules are missing from dependencies

[NekR]

See this: mozilla/sign-addon#111
You have the same problem, modules like when, which and few other are missing from dependencies so had to install them manually.

[kumar303]

I added more info in mozilla/sign-addon#111 (comment) ; your npm is out of date which is causing an installation bug. You will need a newer npm, as stated in the install instructions for web-ext: https://github.com/mozilla/web-ext#installation-from-npm Specifically, npm 3.x or higher is required.