I did this exact same thing but did not see an easy way to reassign the F1 shortcut to toggle the tree style tabs at a quick glance.

I looked through the source code (manifest.json) where it was defined but didn't go looking for how to have the changes seen by firefox, if I were to just change it.

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

Any answers to my laziness would be appreciated!