Pocket web app is a simple way of putting articles found on the internet in your read-it-later list. Although you can save web pages in Pocket for later reading in all the popular browsers through an extension or add-on, but starting from the version 38.0.5 of Firefox, Pocket has become available through a native shortcut in the Firefox toolbar. You do not have to install any extension in Firefox for this. In addition, you can use your Mozilla Firefox account (the one you use to sync Firefox settings across different platforms) for signing in to Pocket. If you already have a Pocket account, then you can use that for signing in as well.
As you launch the new version of Firefox browser, you will find the new Pocket button in the toolbar. Clicking on this button, you can choose to sign-in to your Pocket account. If you do not already have an account, then you can create one using the Mozilla Firefox account or by supplying an email address. In the case you use your Firefox account, you will have to authorize Pocket to use your Mozilla Firefox account.
After you have successfully signed-in to Pocket, you can start using it as usual (just as if you are using Pocket extension for Firefox). Typically, you can click on the Pocket button to save the web page open in current tab to your Pocket account. You can later retrieve this web page from Pocket app on any platform like Android, iPhone etc.
Verdict: Pocket is unquestionably a great app for aggregating interesting articles found online for reading later in your leisure time. Making it the default read-it-later app for Firefox is going to be useful for Pocket users, but it may not be very pleasant to users of other read-it-later apps.