Swell
Open-source Gecko browser with an Arc-style sidebar where each space is a fully isolated identity.
Swell is an open-source browser built by a solo developer as its own Gecko application, not a Firefox reskin, with a small patch queue on Firefox ESR 153 and a Rust core that owns all state. It copies Arc's sidebar model: Favorites, a pinned tree with folders, and a Today list per space, a Cmd+T command bar, and an Arc importer. Its main idea is that each space is a separate identity inside one window, with its own cookies, history, saved passwords, form data, add-on copies, permissions and downloads. In the interface a space cannot share another space's profile. It has no split view, link preview, mini window, boosts, themes, auto-archive, sync, mobile app or AI. It installs only Firefox .xpi add-ons, not Chrome Web Store ones, and has no Widevine DRM or ad blocker. Alpha builds are for macOS on Apple silicon (unsigned) and Linux x86-64 (community build).
29% Arc feature parity (yes = 1, partial = ½).
No AI
No AI features: a stated non-goal in the architecture doc.
github.comNot counted in Arc parity: some people want AI, some avoid it.
None found
No ads, wallet, feed, upsells or account. No telemetry or crash reporter; idle contact is Mozilla Remote Settings and a daily GitHub update check (can be turned off).
github.comAds, crypto, paid upsells, news feeds, default-on telemetry, required accounts.
Organization
| Feature | Swell | Arc | Note |
|---|---|---|---|
| Vertical sidebarTabs live in a vertical sidebar that is the main tab UI and can be hidden. | ✓ | ✓ | Sidebar is the only tab UI (Favorites, Pinned, Today) and hides with a button or Cmd+S.github.com |
| SpacesSeveral workspaces per window, each with its own pinned and open tabs, switched by swipe or shortcut. | ✓ | ✓ | Each space has its own pinned tree and Today list; switch by click, Cmd+1 to 9 or mouse back/forward.github.com |
| Profiles per Space (shareable)Each Space can use its own browser profile (separate cookies, logins, history), and one profile can be shared by several Spaces. | ◐ | ✓ | Each new space gets its own isolated identity; spaces share one only via Arc import.github.com |
| Global favoritesPinned icons at the top of the sidebar shared by every Space (Zen calls them Essentials). | ◐ | ✓ | Favorites belong to the identity, which is one per space, so they are not shared across spaces.github.com |
| Pinned tabs & foldersPer-space pinned tabs that act like bookmarks, with nested folders. | ✓ | ✓ | Per-space pinned tree with nested folders, drag to reorder.github.com |
| Auto-archive tabsUnpinned tabs close automatically after a set time and stay recoverable in an archive. | ✕ | ✓ | Closing a Today tab deletes it; there is no archive or auto-close timer.github.com |
| Space themesPer-space colour or gradient themes. | ✕ | ✓ | Core keeps a tint field but the colour chooser was removed; no per-space themes.github.com |
Browsing
| Feature | Swell | Arc | Note |
|---|---|---|---|
| Split ViewTwo or more tabs side by side in one window. | ✕ | ✓ | No split view in the app; the feature list does not include it.github.com |
| Link previewOpen a link in an overlay without leaving the current page (Zen calls it Glance). | ✕ | ✓ | No link preview overlay; the only peek is the sidebar sliding out on hover.github.com |
| Mini windowA small, chrome-less window for links opened from other apps or a quick search. | ✕ | ✓ | No mini window; links from other apps open as tabs in the main window.github.com |
| Command barA ⌘T palette that searches tabs, history, bookmarks and browser commands. | ◐ | ✓ | Cmd+T bar searches open and pinned tabs plus the space's history; no browser commands.github.com |
| Link routingRules that send links from other apps to a chosen space or profile. | ✕ | ✓ | No routing rules; the macOS bundle declares no http handler, so it cannot be default browser.github.com |
| Site customizationPer-site custom CSS/JS, or zapping elements off a page. | ✕ | ✓ | Boosts are a stated non-goal; only developer tools are built in.github.com |
| Auto picture-in-picturePlaying video pops out into picture-in-picture automatically when you switch tabs. | ? | ✓ |
Compatibility & migration
| Feature | Swell | Arc | Note |
|---|---|---|---|
| Arc importImports Arc spaces, pinned tabs and folders. | ✓ | – | Imports spaces, pinned tabs, folders and favorites from StorableSidebar.json.github.com |
| Chrome extensionsInstalls extensions from the Chrome Web Store. | ◐ | ✓ | Installs only Firefox .xpi add-ons from a file or URL; no Chrome Web Store or .crx.github.com |
| DRM (Widevine)Netflix, Disney+, Spotify web and other DRM video/audio play. Chromium forks need a Google Widevine licence. | ✕ | ✓ | Widevine is not packaged or downloadable (GMP updates off); only ClearKey ships.github.com |
| PasskeysWebAuthn passkeys, Touch ID and the system/iCloud keychain. | ? | ✓ | |
| SyncSpaces, pinned tabs and settings sync across devices. | ✕ | ✓ | Sync is listed as not built yet.github.com |
| Mobile appiPhone, iPad or Android app that shares spaces with desktop. | ✕ | ◐ | Mobile apps are a stated non-goal; desktop only.github.com |
Extras
| Feature | Swell | Arc | Note |
|---|---|---|---|
| Built-in ad blockerAd and tracker blocking without installing an extension. | ✕ | ✓ | Firefox tracking protection is off by default and Swell does not turn it on.github.com |
Extension support
Which extension formats and stores run, beyond the Chrome Web Store row above. Not counted in Arc parity.
| Feature | Swell | Arc | Note |
|---|---|---|---|
| Manifest V2 extensionsThe older extension format Chrome removed in 2025. Needed for full uBlock Origin and other extensions that block requests. | ◐ | ✕ | Gecko runs MV2; Swell's own tabs/windows layer is unfinished (no commands API yet).github.com |
| Manifest V3 extensionsThe current extension format used by the Chrome Web Store (and supported by Firefox and Safari). | ◐ | ✓ | Gecko runs MV3; Swell's own tabs/windows layer is unfinished, so some extensions may break.github.com |
| Firefox add-onsAdd-ons from addons.mozilla.org or .xpi files install and run. | ◐ | ✕ | Only manual .xpi installs (file or URL); no add-on store UI. One copy per space.github.com |
| Safari extensionsSafari web extensions (the App Store extension apps Safari uses) install and run. | ✕ | ✕ | Gecko app; only .xpi WebExtension packages install.github.com |