--- a/devtools/client/debugger/new/test/mochitest/head.js
+++ b/devtools/client/debugger/new/test/mochitest/head.js
@@ -575,18 +575,18 @@ function togglePauseOnExceptions(
  */
 function invokeInTab(fnc) {
   info(`Invoking function ${fnc} in tab`);
   return ContentTask.spawn(gBrowser.selectedBrowser, fnc, function*(fnc) {
     content.wrappedJSObject[fnc](); // eslint-disable-line mozilla/no-cpows-in-tests, max-len
   });
 }
 
-const isLinux = Services.appinfo.OS === "Linux";
-const isMac = Services.appinfo.OS === "Darwin";
+const isLinux = /^gtk/.test(Services.appinfo.widgetToolkit);
+const isMac = Services.appinfo.widgetToolkit === "cocoa";
 const cmdOrCtrl = isLinux ? { ctrlKey: true } : { metaKey: true };
 const shiftOrAlt = isMac
   ? { accelKey: true, shiftKey: true }
   : { accelKey: true, altKey: true };
 
 // On Mac, going to beginning/end only works with meta+left/right.  On
 // Windows, it only works with home/end.  On Linux, apparently, either
 // ctrl+left/right or home/end work.

--- a/devtools/client/inspector/local-toolbox.js
+++ b/devtools/client/inspector/local-toolbox.js
@@ -53,22 +53,22 @@ function fixStylesheets(doc) {
   let themeLink = doc.createElement("link");
   themeLink.setAttribute("rel", "stylesheet");
   themeLink.setAttribute("href", "/devtools/skin/light-theme.css");
 
   doc.head.appendChild(themeLink);
   doc.documentElement.classList.add("theme-light");
   doc.body.classList.add("theme-light");
 
-  if (appinfo.OS === "Darwin") {
+  if (appinfo.widgetToolkit == "windows") {
+    doc.documentElement.setAttribute("platform", "win");
+  } else if (appinfo.widgetToolkit == "cocoa") {
     doc.documentElement.setAttribute("platform", "mac");
-  } else if (appinfo.OS === "Linux") {
+  } else {
     doc.documentElement.setAttribute("platform", "linux");
-  } else {
-    doc.documentElement.setAttribute("platform", "win");
   }
 }
 
 /**
  * Called each time a childList mutation is received on the main document.
  * Check the iframes currently loaded in the document and call fixStylesheets if needed.
  */
 function fixStylesheetsOnMutation() {

--- a/devtools/client/netmonitor/index.js
+++ b/devtools/client/netmonitor/index.js
@@ -61,22 +61,22 @@ window.store = store;
  * Will also add mandatory classnames and attributes to be compatible with devtools theme
  * stylesheet.
  */
 window.addEventListener("DOMContentLoaded", () => {
   for (let link of document.head.querySelectorAll("link")) {
     link.href = link.href.replace(/(resource|chrome)\:\/\//, "/");
   }
 
-  if (appinfo.OS === "Darwin") {
+  if (appinfo.widgetToolkit == "windows") {
+    document.documentElement.setAttribute("platform", "win");
+  } else if (appinfo.widgetToolkit == "cocoa") {
     document.documentElement.setAttribute("platform", "mac");
-  } else if (appinfo.OS === "Linux") {
+  } else {
     document.documentElement.setAttribute("platform", "linux");
-  } else {
-    document.documentElement.setAttribute("platform", "win");
   }
 });
 
 bootstrap(React, ReactDOM).then((connection) => {
   if (!connection) {
     return;
   }
   renderRoot(React, ReactDOM, App, store);

--- a/devtools/client/shared/developer-toolbar.js
+++ b/devtools/client/shared/developer-toolbar.js
@@ -128,20 +128,20 @@ exports.CommandUtils = CommandUtils;
 /**
  * Due to a number of panel bugs we need a way to check if we are running on
  * Linux. See the comments for TooltipPanel and OutputPanel for further details.
  *
  * When bug 780102 is fixed all isLinux checks can be removed and we can revert
  * to using panels.
  */
 loader.lazyGetter(this, "isLinux", function () {
-  return Services.appinfo.OS == "Linux";
+  return /^gtk/.test(Services.appinfo.widgetToolkit);
 });
 loader.lazyGetter(this, "isMac", function () {
-  return Services.appinfo.OS == "Darwin";
+  return Services.appinfo.widgetToolkit == "cocoa";
 });
 
 /**
  * A component to manage the global developer toolbar, which contains a GCLI
  * and buttons for various developer tools.
  * @param chromeWindow The browser window to which this toolbar is attached
  */
 function DeveloperToolbar(chromeWindow) {
@@ -942,24 +942,24 @@ OutputPanel.prototype._resize = function
   // Set max panel width to match any content with a max of the width of the
   // browser window.
   let maxWidth = this._panel.ownerDocument.documentElement.clientWidth;
 
   // Adjust max width according to OS.
   // We'd like to put this in CSS but we can't:
   //   body { width: calc(min(-5px, max-content)); }
   //   #_panel { max-width: -5px; }
-  switch (Services.appinfo.OS) {
-    case "Linux":
+  switch (Services.appinfo.widgetToolkit) {
+    case "windows":
       maxWidth -= 5;
       break;
-    case "Darwin":
+    case "cocoa":
       maxWidth -= 25;
       break;
-    case "WINNT":
+    default:
       maxWidth -= 5;
       break;
   }
 
   this.document.body.style.width = "-moz-max-content";
   let style = this._frame.contentWindow.getComputedStyle(this.document.body);
   let frameWidth = parseInt(style.width, 10);
   let width = Math.min(maxWidth, frameWidth);

--- a/devtools/client/webconsole/local-dev/index.js
+++ b/devtools/client/webconsole/local-dev/index.js
@@ -56,22 +56,22 @@ document.querySelector("#mount").appendC
 document.documentElement.classList.add("theme-light");
 
 // Copied from netmonitor/index.js:
 window.addEventListener("DOMContentLoaded", () => {
   for (let link of document.head.querySelectorAll("link")) {
     link.href = link.href.replace(/(resource|chrome)\:\/\//, "/");
   }
 
-  if (appinfo.OS === "Darwin") {
+  if (appinfo.widgetToolkit == "windows") {
+    document.documentElement.setAttribute("platform", "win");
+  } else if (appinfo.widgetToolkit == "cocoa") {
     document.documentElement.setAttribute("platform", "mac");
-  } else if (appinfo.OS === "Linux") {
+  } else {
     document.documentElement.setAttribute("platform", "linux");
-  } else {
-    document.documentElement.setAttribute("platform", "win");
   }
 });
 
 let consoleFrame;
 function onConnect(connection) {
   // If we are on the main dashboard don't render the component
   if (!connection || !connection.tabConnection || !connection.tabConnection.tabTarget) {
     return;