Arcsidebar 0.5.1

• Tabs nobody has looked at for 45 minutes are unloaded, and load again when
  clicked. A space's tabs stay in memory while the space is hidden, so a window
  was holding every tab of every space at once. Tabs playing audio, tabs in
  private windows and the tab you're on are left alone.

Install: download arc-0.5.1.xpi below and open it in Firefox. Installed copies update themselves.