With https://phabricator.services.mozilla.com/D104391 landed, the only difference between the FrameTargetActor and its base class BrowsingContextTargetActor is the title implementation:

FrameTargetActor:

Object.defineProperty(frameTargetPrototype, "title", {
  get: function() {
    return this.window.document.title;
  },
  enumerable: true,
  configurable: true,
});

https://searchfox.org/mozilla-central/rev/8cb90829ccf18f95cd75d8419c84729c754a8e83/devtools/server/actors/targets/frame.js#54-60

BrowsingContextTargetActor

  get webNavigation() {
    return this.docShell.QueryInterface(Ci.nsIWebNavigation);
  },

  get contentDocument() {
    return this.webNavigation.document;
  },

  get title() {
    return this.contentDocument.contentTitle;
  },

https://searchfox.org/mozilla-central/rev/8cb90829ccf18f95cd75d8419c84729c754a8e83/devtools/server/actors/targets/browsing-context.js#472-491

See initial conversation at https://phabricator.services.mozilla.com/D104391#inline-585657

After some tests I haven't found any situation where the BrowsingContextTargetActor's implementation of title could return a valid title?
I tested:

• webextension targets
• parent process targets
• chrome window targets

The title getter was added 7 years ago in https://bugzilla.mozilla.org/show_bug.cgi?id=937172, but I am not sure it ever worked? contentTitle doesn't seem available on document: https://searchfox.org/mozilla-central/source/dom/webidl/Document.webidl

contentTitle only seems available on browser elements.

Maybe this was meant to point to document.title?