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Install from Firefox Add-ons

Install most Firefox extensions directly from their addons.mozilla.org pages. Compatibility varies where an extension depends on a Firefox-only API or a native companion.

Desktop today

The current extension installation and management interface is available in Crest for Mac.

Firefox extensions are often a close fit for Crest. Crest's engine runs the promise-based browser.* APIs Firefox extensions are written against, and it supports Manifest V2 extensions with a persistent background page.

Before you add it​

Choose the Space and device that should own the extension. The package, enablement, permissions, storage, pinning, and website access remain independent from every other Space and device.

Install the extension​

  1. In Crest on Mac, open the extension's page on addons.mozilla.org.
  2. Select Add to Crest, below the page's own install button.
  3. Wait while Crest downloads the add-on from Mozilla and checks it against the checksum, size, and identity Firefox Add-ons published for it.
  4. In Crest's native review sheet, confirm Install In shows the intended Space.
  5. Open Review Access and Compatibility. Read the requested Permissions, Website Access, and any WebKit Compatibility Warnings.
  6. Select Add Extension.
  7. When Crest confirms the extension was added, select Done.

The extension is enabled only in the Space and device shown during review. To change it later, open Crest Settings → Extensions and choose that Space.

Use it after installation​

  • Open Site Controls beside the address field to find the extension’s action. Pin the action if you want it in the active Space’s extension strip. See Pin extensions and use popups.
  • Open Crest Settings → Extensions, choose the Space, and expand the extension to turn it on or off, open its options page, review status, or remove it.
  • Review or change Permissions and Website Access independently for each Space. See Manage permissions and website access.
  • If the extension declares commands, assign them under Crest Settings → Shortcuts. See Set extension keyboard shortcuts.

Get a newer version​

Crest’s automatic updater currently covers Chrome Web Store installations. To refresh a Firefox extension, reopen its Firefox Add-ons listing in Crest, select Add to Crest, review the current version, and install it into the same Space. Crest verifies the new package against Mozilla’s listing before replacing the installation.

What Crest checks before installing​

Crest resolves the listing through Mozilla's public add-ons API and then refuses anything that does not match it exactly:

  • The download must come from addons.mozilla.org over HTTPS, and must still be on that host after any redirect.
  • The downloaded file must reproduce the SHA-256 checksum and byte count Mozilla published for that exact version.
  • The archive must carry Mozilla's signature files. A hand-built or side-loaded .xpi is refused.
  • If the add-on declares its own Firefox add-on ID, it must match the ID on the listing.

Only listings for extensions are eligible. Themes, dictionaries, and language packs are not WebExtensions Crest can host, so Add to Crest does not appear on their pages.

What is not supported​

Some Firefox capabilities have no equivalent in Crest's engine. An extension that depends on one of them still installs, but that feature will not work:

  • Sidebars (sidebar_action)
  • Container tabs (contextualIdentities)
  • Browser themes applied by an extension
  • Request blocking and rewriting through blocking webRequest, which is how some older content blockers work

Crest lists what it detected under WebKit Compatibility Warnings in the review sheet before you install, and keeps the same explanation in Extensions settings afterward.

Installation is not a universal compatibility promise

A package can verify, install, and start its background script while a later account, helper, or site workflow remains limited. Crest keeps known limits separate from unexpected Needs attention runtime failures.

Native companion apps are not available​

Crest verifies native companion access for Chrome Web Store extensions only, because that path can match an extension's verified store identity against a registered host's permitted origins. A Firefox add-on that requests nativeMessaging cannot be installed; Crest explains this before installation. If the extension also ships a Chrome Web Store version, install that version in a direct build of Crest instead. See Native companion limits.

Next: Manage permissions and website access.