Firefox WebExtensions and Chromium Manifest V3 interoperability at the manifest boundary

Firefox release beta and ESR channels shipped in late 2024 support both Manifest V2 and Manifest V3 extensions. Manifest V3 became the default for new submissions starting with Firefox 109 (January 2023) and the manifest version field in manifest.json is accepted without errors in the release channel.

Chromium Manifest V3 mandates the declarativeNetRequest API for network interception and disallows the legacy webRequest.onBeforeRequest listener. Firefox Manifest V2 still permits blocking via the webRequest API including conditional and regex-aware listeners. In Firefox Manifest V3 the declarativeNetRequest API is provided but implements a restricted feature set: only static rule types are supported the number of rules and rule complexity is limited and dynamic rule modification or regex capture groups are not available.

Mozilla has not published a fixed end-of-support date for Manifest V2 in Firefox; the roadmap indicates an intention to deprecate V2 once V3 adoption reaches sufficient ecosystem coverage but the timeline remains open and may be extended based on extension maintainer feedback and channel-specific factors.

To verify your own extension’s compatibility run about:support in Firefox to confirm the release channel and version and inspect the manifest.json version field for errors during about:addons loading and test the extension with a minimal declarativeNetRequest rule set comparing supported rule types against the MDN documentation for Firefox’s declarativeNetRequest. If your extension uses regex-based redirects or dynamic rule updates what Firefox release channel are you targeting and does your manifest specify version 2 or 3? This detail determines whether a dual-manifest approach or a full V3 migration is viable for your use case.