I removed the TorButton Firefox plugin. When I reinstall it, it's remembers the previous settings.
This plugin is not listed in about:config when it is installed. The other plugins are there. So, I don't know the path to the plugin. I don't think that single .dll file would solve the problem anyway.
I deleted the directory under Vidalia where a TorButton folder is created. It's still remembers the preferences.
Here's the reason I need to completely remove it. There is a bug with Torbutton, and the customize toolbar does not work with it installed. You have to disable it, customize the toolbar, and then re-enable it. The problem is, once you remove torbutton, when you add it back, the button is not on the toolbar. You have to use the customize option to put it there - which is disabled. If you add torbutton to a fresh install of firefox, the button is placed on the toolbar automatically. I'm hoping that by being able to completely remove torbutton, it can be re-installed without having to reinstall firefox. As it is, if a new plugin is added, and the toolbar needs customized, I have to reinstall firefox, and make Torbutton the last addon added.
UPDATE: Resetting Firefox (Help->Troublinshooting info->...) does not solve the problem. I have to completely uninstall and reinstall Firefox.