Firejail is based on the same type of tech as containers, namely process restrictions that you can activate on a per namespace basis. The MS sandbox is a separate VM with its own sandboxed kernel, which sounds like stronger isolation on the face of it.

You are right but I stand my point, the main functionality (isolation) is the same. One may indeed prefer a stronger isolation and be ready to accept a larger memory footprint, less easy ways to fine-tune isolation in order to let the isolated software access to some system-wide resources...