If you open the Inspector or the Style Editor panel while a page is loading, the panel stays empty and I get the following error in the Browser Console: "Protocol error (unknownError): [Exception... "Unexpected error" nsresult: "0x8000ffff (NS_ERROR_UNEXPECTED)" location: "JS frame :: resource://gre/modules/commonjs/toolkit/loader.js -> resource://devtools/server/actors/highlighters/utils/markup.js :: CanvasFrameAnonymousContentHelper.prototype._insert :: line 236" data: no]"1 Promise-backend.js:937 Steps to reproduce: 1. Go to https://www.mozilla.org/en-US/ 2. While the page is still loading press open the Inspector (Ctrl+Shift+C) This only seems to happen for the Inspector and the Style Editor panel. I also tried it with the Console, Debugger and Network panel, though they work as expected. Sebastian

For the inspector-panel, this is bug 1151909. For the style-editor, this is bug 1224558 comment 1. Going to duplicate this with bug 1151909.

Both of these bugs are potentially complex. We've had the bug for the inspector-panel opened for as long as I can remember working on the team. Sad that we've never found the time or energy to work on it ... I might take a look at the style-editor bug.

Thank you for pointing me to the right bugs. Sebastian