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

Integration boundary

We maintain a cross-browser extension built on the WebExtensions API (browser.* / chrome.*) that currently ships as Manifest V2. Firefox supports MV3 while also committing to continued MV2 support, whereas Chromium is moving to deprecate MV2. We need to decide how to structure the codebase across this boundary.

Constraints and uncertainty

Our extension relies on blocking WebRequest for request filtering, which Firefox retains in MV3 but Chromium restricts in favor of declarativeNetRequest. We also ship an internal build that must pass Firefox's signing requirement, and we target both Release and ESR channels, where API availability may differ. Some Chrome-only namespaces we use have no Firefox equivalent, so we already do capability detection.

Target assumption: current Firefox Release and ESR as of 2026; exact MV2 longevity commitments are version-sensitive and need verification against Mozilla's current documentation.

Questions

1. Is maintaining a single MV3 manifest with graceful degradation for blocking WebRequest viable across both Firefox and Chromium, or is a dual-manifest build the safer long-term structure?
2. Does Firefox's MV2 support commitment apply equally to Release and ESR, and is there a documented signal for when that might change?
3. Are there known MV3 API behaviors in Firefox (beyond WebRequest) that diverge from Chromium enough to require per-browser code paths?