For users who don’t know personalizing Firefox or for that matter, don’t know what add-ons are or who visit add-ons manger for the first time, Mozilla has redesigned the add-on discover pane and shipped with Firefox 48, check Get add-ons page in add-ons manager at about:addons to see it in action.

The discovery page comes with a list of curated extensions and themes (Mozilla feels they can display the right ones in front of the user) and makes it easy for the users to install their first add-on. They can find more add-ons by clicking on the link placed at bottom, which leads to AMO.
Read more about new disco pane here

Disabling the new add-on discovery page in Firefox 48

With the new design, when you select ‘Get add-ons’ in about:add-ons page, Firefox connects to Mozilla’s new domain https://discovery.addons.mozilla.org and fetches the content to display in your browser (internet connection is required). While this also happens in the case of old one, but if you don’t like the new design, you can certainly revert back to the previous get add-ons page design, here is how.

Firefox 48 new to old add-ons pane

1 . Visit about:config
2. Type Discovery in the search box, in the search results displayed, select extensions.webservice.discoverURL, and change it to
https://services.addons.mozilla.org/%LOCALE%/firefox/discovery/pane/%VERSION%/%OS%/%COMPATIBILITY_MODE%
(copy and paste).
3. To get the new discovery pane back, right click on extension.webservice.discoverURL and select Reset this will change preference value
https://discovery.addons.mozilla.org/%LOCALE%/firefox/discovery/pane/%VERSION%/%OS%/%COMPATIBILITY_MODE%

4. Done. Happy add-on discovering!