Try starting Firefox in a brand new profile. Don't modify any settings and don't install any add-ons besides Adblock Plus.https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
Do you still have the same issue? Either way, in your regular profile, type about:support
in the address bar and press Enter. Copy the entire list of extensions to the clipboard and paste it in a reply here.