I came across the logic below in Document::InitCSP, which checks whether a node principal is an ExpandedPrincipal (and attaches a CSP to it when it is). A node principal can never be an expanded principal, so the branch is never reached. We should remove it (which would also simplify part of the work on bug 1548468).

https://searchfox.org/mozilla-central/rev/7fe9421af35256a95acc4620e9e0b76df7867287/dom/base/Document.cpp#3675-3682

nsresult Document::InitCSP(nsIChannel* aChannel) {
  ....

  nsCOMPtr<nsIPrincipal> principal = NodePrincipal();

  ...

    // Bug 1548468: Move CSP off ExpandedPrincipal
    // Currently the LoadInfo holds the source of truth for every resource load
    // because LoadInfo::GetCSP() queries the CSP from an ExpandedPrincipal
    // (and not from the Client) if the load was triggered by an extension.
    auto* basePrin = BasePrincipal::Cast(principal);
    if (basePrin->Is<ExpandedPrincipal>()) {
      basePrin->As<ExpandedPrincipal>()->SetCsp(mCSP);
    }

When working on Bug 1548468 we should also address this bug, putting in the backlog for now.

https://hg.mozilla.org/mozilla-central/rev/43839ffc2ae0