That got me thinking that the same is true for other sidebars, as the View > Sidebar menu seems like a relatively obscure and cumbersome way to access them. My inclination was to place a permanent interface on the left-hand side of the browser window, right where sidebars appear, that would be contextual, discoverable, and easily accessible.
Tabs seemed to fit these requirements and are consistent with Firefox's "one sidebar open at a time" behavior, so, with some helpful input from Dan Mills on how similar interfaces in other applications have behaved, I cooked up an experimental extension that implements a column of vertical tabs representing sidebars down the left-hand side of the browser window.
Dragging a tab to the right opens the corresponding sidebar, while dragging it back to the left closes it. Dragging also lets you set the sidebar width. Or open a sidebar to its previous width by simply clicking its tab (clicking again toggles it closed).
The extension supports both built-in sidebars (Bookmarks, History) and those provided by extensions, as long as the extensions use the standard machinery for defining their sidebars. And it extends that machinery with support for tab icons: just set the image attribute on your
It uses the new -moz-transform CSS property to orient text vertically on the tabs (and a bit of the new drag-drop API to start the drag operation) so it only works on recent nightlies. And it's a prototype, so there's still some wonkiness in its appearance and behavior.
Check it out in the AMO sandbox, and let me know what you think. (Also note the All-in-One Sidebar, another take on the same concept.)