Browser-specific manifest fields#

Avoid maintaining separate manifest files for every browser.

Extension.js lets you define browser-specific values inline with prefixes, then emits only the fields that match the active target at compile time.

Why this matters#

Browsers still differ in key manifest areas, like background configuration and vendor metadata. Prefixed fields let you keep one source manifest.json while producing browser-correct output for Chromium and Firefox families.

How it works#

Extension.js scans manifest keys and resolves prefixed entries for the selected browser:

• Chromium family targets (chrome, edge, chromium-based) resolve: chromium:, chrome:, edge:
• Gecko family targets (firefox, gecko-based) resolve: firefox:, gecko:

When a prefixed key matches the active target, Extension.js rewrites it to the unprefixed key in the emitted manifest.

For Chromium-based browsers (Chrome, Edge, ...)#

{
  "chromium:background": {
    "service_worker": "sw.js"
  }
}

For Firefox#

{
  "firefox:background": {
    "scripts": ["sw.js"]
  }
}

You can also target vendor-specific variants inside the same browser family:

{
  "chrome:action": {
    "default_title": "Chrome variant"
  },
  "edge:action": {
    "default_title": "Edge variant"
  }
}

This makes service_worker available only for Chromium-family outputs while keeping background.scripts for Firefox outputs.

Supported prefix map:

This works for any manifest field at any level, including permissions, content_scripts, and background.

Best practices#

• Keep shared defaults unprefixed: Put common fields in regular manifest keys, then prefix only browser-specific differences.
• Prefix only when behavior diverges: Use browser prefixes when runtime requirements differ.
• Build per target in CI: Generate and verify each browser output (dist/<browser>) to catch compatibility regressions early.
• Validate with MDN: Use MDN Web Docs to confirm support before adding browser-only settings.

Next steps#

• Learn more about the Browsers Available.
• Learn more about Cross-Browser Compatibility.