Blockable items list has been replaced by a Developer Tools panel - press Ctrl+Shift+I (Cmd+Option+I on macOS) to open Developer Tools, then switch to the Adblock Plus panel.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. 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