Mozilla Firefox has "system addons" which cannot be removed and work like an integrated feature of Firefox. In Firefox Nightly, a new system add-on appears to be installed out-of-the-box. Besides Pocket and Hello, an extension called "FlyWeb" now comes with the browser.
System add-ons are special add-ons which are not visible in the browser's extension manager. To see such add-ons, you need to type the following in the address bar:
There, you need to scroll down to the section named Extensions. See the following screenshot:
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