User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:57.0) Gecko/20100101 Firefox/57.0 Build ID: 20171130143849 Steps to reproduce: - Clone https://gist.github.com/twolfson/c47ab162b73fd86a59e05ac91b455108 - Open Firefox and navigate to `about:debugging` - Add our extension as a temporary add-on - Open its "Debug" console - Navigate to a normal website (e.g. <https://mozilla.org/>) - Open our in-page Developer console - Click our icon, look at "Debug" console, and verify message sending - Look at our in-page Developer console and verify message logged - Enter Responsive Design Mode - Click our icon, look at "Debug" console, and verify message failing to send - Look at our in-page Developer console and verify no message logged Actual results: No message was sent from background script (`global.js`) to content script (`injected.js`). Instead, we got error from `undefined`: "Error: Could not establish connection. Receiving end does not exist." Expected results: Message should be sent from background script (`global.js`) to content script (`injected.js`) and logged in in-page Developer Console