Blockable items list has been replaced by a Developer Tools panel - press Ctrl+Shift+I (Cmd+Option+I on macOS) to open Developer Tools, then switch to the Adblock Plus panel. A more obvious way to open it would be great but sadly currently not possible (we filed a Firefox bug on that a year ago). You might need to reload the page, requests performed before the panel was opened aren't being displayed. As to the user interface, we will keep improving it and adding missing functionality back. You can read up more on the background here: https://adblockplus.org/blog/the-plan-towards-offering-adblock-plus-for-firefox-as-a-web-extension