Note that the shortcut is equivalent to hitting the "Tree Style Tab" button in the main toolbar, so if you're like me and want to show the tabs all the time you don't need to worry about having a keyboard shortcut.

If you do need the shortcut, AFAIK your only option is to edit the extension. Modify the manifest.json file in the root of the XPI around:

  "commands": {
    "_execute_browser_action": {
      "suggested_key": { "default": "F1" },
      "description": "__MSG_sidebarToggleDescription__"
    }
  }

Unfortunately, actually installing your changes requires a decent amount of arcane incantation (particularly if you're not using Nightly) -- https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Pu... describes how to do it if you want to go that route.