Yes, that sounds exactly right. But I would add that there is a technical reason for the more limited blocklist setup, in that it's much more reasonable to do a simple list check asynchronously than to run JS code.

But to me, that just suggests that firefox might want to consider an additional async API. I can think of many options that are more powerful than what Chrome is moving to. eg use the exact logic uBO needs. Or register a WASM function with no access to anything that would problematic to run async.