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/917718d991e7e537fa1bc3b6d39cb3f0 - 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/>) - Refresh our page and look at our "Debug" console - Click our icon, look at "Debug" console, and verify tab ids correspond - Enter Responsive Design Mode - Refresh our page (in Responsive Design Mode) and look at our "Debug" console - Click our icon, look at "Debug" console, and verify tab ids don't correspond Actual results: We expected a web extension receiving `runtime.onMessage` in Responsive Design Mode to contain `sender.tab` Expected results: A web extension receiving `runtime.onMessage` in Responsive Design Mode did not contain `sender.tab`

Here's a sample output in normal mode: ``` tab Object { id: 27, index: 2, windowId: 3, highlighted: true, active: true, pinned: false, status: "complete", discarded: false, incognito: false, width: 1920, 7 more… } global.js:2:3 tab.id 27 sender Object { id: "87d0f52383394464ef06284b8d59c243000…", frameId: 0, url: "http://localhost:8080/", envType: "content_child", extensionId: "87d0f52383394464ef06284b8d59c243000…", contextId: "150-8", tab: Object } sender.tab Object { id: 27, index: 2, windowId: 3, highlighted: true, active: true, pinned: false, status: "complete", discarded: false, incognito: false, width: 1920, 7 more… } global.js:8:3 sender.tab.id 27 ``` and here's a sample output in Responsive Design Mode: ``` tab Object { id: 27, index: 2, windowId: 3, highlighted: true, active: true, pinned: false, status: "complete", discarded: false, incognito: false, width: 320, 6 more… } global.js:2:3 tab.id 27 sender Object { id: "87d0f52383394464ef06284b8d59c243000…", frameId: 0, url: "http://localhost:8080/", envType: "content_child", extensionId: "87d0f52383394464ef06284b8d59c243000…", contextId: "153-8" } global.js:7:3 sender.tab undefined ```

Recategorized as per https://bugzilla.mozilla.org/show_bug.cgi?id=1425660

Can you try Firefox Beta or Nightly to if the issue is resolved there? Some WebExt improvements around RDM were added in 58 and later.

I tried out Firefox Beta @ 58.0b13 and it looks like the issue has been resolved. Sorry for the trouble and thanks for your time!

Thanks for the update, that's great to hear! :)