Firefox already supports Manifest V3. Crucially, though:

• Firefox continues to support Manifest V2 for the foreseeable future.
• Firefox will not adopt the arbitrary limitation of content/ad blocking rules in Manifest V3, which is what’s bad about it.
• Firefox offers APIs in addition to Manifest V2 and V3, with which more powerful extensions can be built. This is why uBlock Origin has been better on Firefox for quite a while already.