Small improvements to finding and querying iframes:
- Use documentURI instead of baseURI in sidebar.js, as that is what the 'frameURL' option to eval() uses.
- Keep track of the parent frame's URI when it is provided via postMessage and use that to provide the targetOrigin parameter to postMessage when posting messages to the parent frame.
- Don't ignore frames which are considered hidden from accessibility.
- Remove the URIs of frames which receive a 'beforeunload' event.