Firefox WebExtensions and Chromium-style MV3: how long can one extension target both manifest formats?

Answer

You can keep a single Manifest V3 package only if the extension’s blocking WebRequest logic can be replaced by declarativeNetRequest (or omitted) and you accept the reduced filtering capability in Firefox. Otherwise, maintaining two separate manifests—one MV2 for Firefox and one MV3 for Chromium—is the safer long‑term approach.

Confirmed facts

• Chromium blocks the use of blocking WebRequest in Manifest V3; Firefox still allows it in MV3.
• Firefox has stated an intention to continue supporting Manifest V2 while Chromium removes it, but no fixed end‑of‑life date is published.
• Manifest V3 differences go beyond WebRequest (service‑worker background, action API, host‑permission handling, declarativeNetRequest limits).
• Internal builds must still satisfy Mozilla signing; an unsigned package fails regardless of manifest version.

Likely explanation

Because the strongest request‑filtering capability you need (blocking WebRequest) cannot be expressed in a Chromium‑compatible MV3 manifest, a single manifest would force you to downgrade to declarativeNetRequest on both browsers. That loses the fine‑grained blocking you currently rely on in Firefox. Keeping the MV2 manifest for Firefox preserves the full capability while Chromium moves to MV3, so the extension can evolve independently on each platform.

Steps needed for this case

1. Audit whether any blocking WebRequest rules are essential; if they can be expressed as declarativeNetRequest, proceed to a single MV3 manifest.
2. If blocking WebRequest is required, create two build targets:
• manifest_v2.json for Firefox (Release and ESR).
• manifest_v3.json for Chromium (stable and enterprise).
3. Keep shared JavaScript behind capability detection (browser vs chrome) and expose a common API surface.
4. For each target, run the Mozilla signing process (web-ext sign) and verify installation on Firefox Release and ESR.
5. On Chromium, load the unpacked MV3 package or publish to the Chrome Web Store and test the declarativeNetRequest rules.
6. Compare network‑filtering outcomes (blocked, redirected, upgraded) between the two builds to confirm behavioral parity where possible.

Missing diagnostic detail

To decide whether you can eventually drop the MV2 build, you need the announced end‑of‑life date for Manifest V2 in Firefox Release and ESR. If that date is known and far enough in the future, you may plan a migration; otherwise, retain the dual‑manifest strategy.