Actually, the link says
> Files submitted for signing will go through an automated review process. If they pass this review, they are automatically signed and sent back to the developer. This process should normally take seconds
You may be thinking of a different type of review process, the signing one sounds almost instantaneous.
This seems to be part of Mozilla's effort to be more like the Apple and Google stores.
Mozila AMO - Learn to embrace the pain.
Addons are running in the chrome context and are thus pretty powerful. It's trival to compromise the whole computer if they aren't reviewed.
Reviewing extensions is critical to their user-experience. If this really doesn't have an team of paid staffers, that's unfortunate.
This can be mitigated by having more volunteers (or paid staff (or both)) to help though.
Nobody knows what it checks for or how it works.