How do I even open the sidebar that's built into Firefox?

Press ctrl+h to open sidebar with the history pane, and ctrl+b to open it with the bookmarks pane open. You can also go into Customize mode and drag the sidebar icon into your toolbar so you can just click it instead.

but clicking on the left edge of the screen does nothing

I linked it more as a proof-of-concept and not as a direct substitute. Try this code instead:

#sidebar-box:not(:hover) {
  width: 1px !important;
  height: 100% !important;
  position: fixed !important;
}
#sidebar-box:not(:hover) > #sidebar-header > toolbarbutton:not(.active) {
  display: none;
}
#sidebar-box:not(:hover) > #sidebar-header {
  padding: 0px;
}

This code assumes that you have your sidebar set to open on the left and the sidebar is currently open. The one downside I found is that clicking the button that indicates the current sidebar and hovering into its menu makes for a pretty poor experience. Unfortunately, this can't be fixed with CSS.