User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:32.0) Gecko/20100101 Firefox/32.0 Build ID: 20140831201947 Steps to reproduce: 1. Make sure you have at least one debug-able addon installed. 2. Make sure addon debugging is allowed 2.1 Open developer tools (Ctrl+Shift+k) 2.2 Settings tab, gear icon 2.3 Tick 'Enable chrome and addon debugging' in 'Advanced settings' section 2. Open about:addons 3. Click 'Debug' on arbitrary addon 4. Click 'Cancel' on 'Incoming connection' dialog (Empty window (uninitialized debugger) remains visible) 5. Click 'Debug' on arbitrary addon Actual results: Nothing happens. If you run firefox from console, following message appears in console: DBG-SERVER: Could not start debugging listener on '6080': [Exception... "Component returned failure code: 0x804b0036 (NS_ERROR_SOCKET_ADDRESS_IN_USE) [nsIServerSocket.initSpecialConnection]" nsresult: "0x804b0036(NS_ERROR_SOCKET_ADDRESS_IN_USE)" location: "JS frame :: resource://gre/modules/commonjs/toolkit/loader.js -> resource://gre/modules/devtools/server/main.js :: DS_openListener :: line 463" data: no] If the empty window is closed, everything gets ok. Expected results: The empty debugger window should be automatically closed after the connection is refused in step 4 of Steps to reproduce.

The same kind of thing happens if you use the browser toolbox in Firefox instance A, and then try to use it in Firefox instance B, too. No feedback at all - would be good if there was /some/ kind of feedback, at a minimum something like "The port we want to listen on (xxxx) is in use." with suggestions of how to remedy...

This should be resolved by Bug 1392744, as the Browser Toolbox picks a random open port now.