Move your Arc sidebar into Zen.

Arc is in maintenance. Zen is the closest thing to it. arc2zen reads Arc's own sidebar file and writes you a script that rebuilds your spaces, favorites, folders and pinned tabs on the other side.

Your file is read in this page and never sent anywhere. This page cannot make network requests at all: its content security policy blocks them. Turn off your wifi and everything still works.

Read this before you start

You will paste code into a privileged console.: Zen warns you there for a reason: pasted code can do anything your browser can do. The whole script is shown below, in full. Read it, or have someone you trust read it, before you run it.
Use a fresh Zen profile.: The script refuses to run on a profile that already holds workspaces, pinned tabs, folders or essentials, so it cannot overwrite the setup you already have. Create a profile in about:profiles and run it there.
Back up your Zen profile first.: cp -R ~/Library/Application\ Support/zen ~/Desktop/zen-backup

01 Find your Arc file

~/Library/Application Support/Arc/StorableSidebar.json

Open Finder.
Press Cmd Shift G.
Paste the path and press Return.

Arc keeps two files side by side. You want StorableSidebar.json, not StorableArchiveItems.json.

02 Drop the file here

Drag StorableSidebar.json in or click to choose it

03 Check what arc2zen understood

Here is what a rebuilt sidebar looks like. Drop your own file and this becomes your sidebar, with the names you can still change.

Space A container · from Arc profile “default”

Essentials, shared by this container

EEssential one
EEssential two
EEssential three

▸Top foldercode
- ·Tab in the top folder
- ▸Nested foldercloud
- ↻Live folderlive · github
·Bare pinned tab

04 Copy the script

Every address in it comes from your own Arc sidebar. Nothing else is added.

Drop your file first

The generated script will appear here once you drop your Arc file.

05 Run it in Zen

Open a fresh Zen profile from about:profiles.
In that profile, open about:config and set devtools.chrome.enabled to true.
Open the Browser Console with Cmd Shift J, or from the menu: Tools, then Browser Tools, then Browser Console.

Not Cmd Opt I. That one is the page inspector, it runs without privileges, and the script will stop with Services is not defined.
Paste the script and press Return.

If the console refuses the paste, type allow pasting there first, then paste again.
Watch the lines prefixed [arc2zen]. When it says done, restart Zen and check that everything is still there.

06 What you still have to do by hand

Sign in again everywhere. Firefox cannot import Chromium cookies, so no session comes across.
Reinstall your extensions from addons.mozilla.org. Their settings do not follow.
Boosts, Easels and Split View have no equivalent in Zen.
Unpinned tabs are left out on purpose. In Arc those are throwaway tabs.

The README covers each of these, and says what was established by reading Zen's code rather than assumed.