The current way sidebars are set up is crappy. My support is for tabs + docking.
A possible interface is to give each sidebar a tab (one for Histroy, Bookmarks, Document Map). This is very simple, and would make all sidebars easily discoverable and accessible.
Icons, text, or icons and text? What's the best default?
A combo-box holding each sidebar is used in many GTK+ applications (Evince, Totem). Disadvantage of not being entirely evident of all available sidebars, and that context menus are not available for individual sidebar components.
The tree view used in Opera and Seamonkey really sucks. It jolts the other sidebar activators up and down when you expand one, and eats up vertical space with each added sidebar.
This is not in competition to the above methods, but a compliment. Basically, a DnD interface would allow you to vertically tile several sidebars (2-3), with and arbitrary-height divider. (Drag up/down, and remember that position.)
It is also a possibility also to make individual sidebar dialogs un-dockable, e.g. place them in thier own seperate window, or place several of them together in thier own window. This leads to the problem of UI association when multiple browser windows are open — GIMP handles this by associating with the most recently focused image window.
The docking method would require a slight change in terminology. The sidebar would refer to the (potentially empty) side-aligned column that would recieve dragged dialogs or docks, individual components that could be added to or modified by extensions.