Regular Adblock was causing memory problems, so I ran its uninstall. I got a popup that it couldn't remove itself and I would have to do it. Rebooting foudn that it wasn't in the Mozilla menu any more, but I still can't install Adblock Plus.
How should I proceed?
Uninstall of Adblock regular
Was that with the latest release 0.5.11.1? If it does install then it's recommended to run the deinstallation procedure on that version to remove the old entries and install again. If the latest version of Adblock Plus doesn't allow you to over-install then try browsing to your profile folder and remove "adblock.jar" from the chrome directory. Ignore the errors you see on startup and then try installing Adblock Plus.
http://kb.mozillazine.org/Profile_Folder#Mozilla_Suite
http://kb.mozillazine.org/Profile_Folder#Mozilla_Suite