FCKConfig.BrowserContextMenu. When this option is set to true, browser's context menu is available by Right click. Editor's context menu doesn't appear.
Note: if editor's context menu isn't available for you or browser's context menu appears at the same time with editor's context menu, please check the browser's preferences. For FF3: Edit -> Preferences -> Content tab ->Advanced... button -> Allow scripts to: Disable or replace context menus (this checkbox should be checked).