You can read up on the background here: https://adblockplus.org/blog/the-plan-towards-offering-adblock-plus-for-firefox-as-a-web-extension. As to blockable items, this was replaced by a Developer Tools panel - press Ctrl+Shift+I (Cmd+Option+I on macOS) to open Developer Tools, then switch to the Adblock Plus panel. You might need to reload the page, requests performed before the panel was opened aren't being displayed.