This is an extension for SeaMonkey that adds an "Add Custom Tab" option to the "Tabs" dropdown menu on the sidebar and to the right-click menu for each sidebar header. Clicking "Add Custom Tab" brings up a dialog that allows you to enter a name and URL, and optionally make the tab persistent (always running).
Download from AMO: https://addons.mozilla.org/en-US/seamonkey/addon/sidebar-add-custom-tab/