> While other browser vendors introduced declarativeNetRequest (DNR) in favor of blocking Web Request in MV3, Firefox MV3 continues to support blocking Web Request and will support a compatible version of DNR in the future. We believe blocking Web Request is more flexible than DNR, thus allowing for more creative use cases in content blockers and other privacy and security extensions. However, DNR also has important performance and compatibility characteristics we want to support.
https://blog.mozilla.org/addons/2022/11/17/manifest-v3-signi...
So, in a nutshell, they won't deprecate an existing working API, like Google did. But they will additionally support the new API.