Frame selector was implemented in http://trac.webkit.org/changeset/91839, that did not cover the cd(frame) though.
I have no idea what this is describing. Please re-open if it still makes sense and more details are available.
This is talking about the function described here: <http://getfirebug.com/wiki/index.php/Command_Line_API#cd.28window.29>. It lets you change the default evaluation context of the console from the main frame to a subframe.
Ah, the inspector now has a selector next to the console that lets you change the evaluation context. Is this still worth implementing? I've never heard of it. (Is there any way to figure out what Firebug command line APIs are actually used?)
You mean which APIs developers actually rely on? You'd probably have to ask the Firebug folks. I don't think we track API usage in WebKit.