You've been very patient with me so I don't want to appear ungrateful. Having done more research I'm now able to 'close the loop'.
Super AdBlocker installs with a plugin for Firefox (i.e., it's not a separate download). You can choose, from SAB's interface, whether you want this plugin to be enabled. When I uninstalled SAB the uninstaller did not remove the plugin, which remained in the Super AdBlocker folder in Program Files.
Firefox can search for plugins in a program's own folder - that's where it found the plugin. Of course, without the program the plugin was useless.
I would never have known about this were it not for the new FF 3, which now displays all its plugins. It was a simple matter of deleting the plugin (two small files) from FF's plugins folder and, voila, problem solved.
(I must have manually removed the Super AdBlocker folder in Program Files some time ago.)
To summarize: I've taken up your time with something that has nothing to do with AdBlock Plus and which I would have figured out on my own with a little more patience.