To be more accurate: Mozilla does plan on deprecating MV2 once they have all of the MV3 stuff supported and sufficient time to transition has been given but they will make the the crucial “webRequestBlocking” API used by ad blockers available on MV3 (unlike Chrome) for those extensions that need it to do more than declarativeNetRequest allows for.

See: https://blog.mozilla.org/addons/2022/11/17/manifest-v3-signing-available-november-21-on-firefox-nightly/

Towards the end of 2023 — once we’ve had time to evaluate and assess MV3’s rollout (including identifying important MV2 use cases that will persist into MV3) — we’ll decide on an appropriate timeframe to deprecate MV2.