thought: I would really like to stop exposing this. If we had to give it an accurate name, it's really "getParentProcessTargetActorNotAvailableViaTargetHelpers", which is hardly a very useful API outside of the few methods in this class.

The only other call site is the parent-process storage actor/resource which uses it as follows:

const parentProcessTargetActor = this.watcherActor.getTargetActorInParentProcess();
const {
  browsingContextID,
  innerWindowId,
} = parentProcessTargetActor.form();
await this._spawnActor(browsingContextID, innerWindowId);

I think that something we quickly discussed during the review for the storage resources refactor. We can probably switch to something else here, because it's a bit artificial to get IDs for the top level target of a browser toolbox session.

question: Do you know if we have webExtensionTarget actors running in the parent process?

About storage, yes, I think there might be way to use browsingContextID set to -1 so that ResourceCommand automagically associate the resource to the top level, parent process target.

This is being used by network event watcher only for now.

Hum... good question... you are probably right. This method is probably really only for the browser toolbox...

Somehow the webextension top target, created from the Descriptor, in the webext process via message manager is somehow managed by DevToolsFrameChild.
At least browsing-context-helpers filrting method are defined to do allow this.