diff --git a/content/install.js b/content/install.js
index a20ff50a..e6810f9c 100644
--- a/content/install.js
+++ b/content/install.js
@@ -1,9 +1,9 @@
 'use strict';
 
-const CHROMIUM = /Chromium/.test(navigator.userAgent); // non-Windows Chromium
-const FIREFOX = /Firefox/.test(navigator.userAgent);
-const VIVALDI = /Vivaldi/.test(navigator.userAgent);
-const OPERA = /OPR/.test(navigator.userAgent);
+const CHROMIUM = chrome.app && /Chromium/.test(navigator.userAgent); // non-Windows Chromium
+const FIREFOX = !chrome.app;
+const VIVALDI = chrome.app && /Vivaldi/.test(navigator.userAgent);
+const OPERA = chrome.app && /OPR/.test(navigator.userAgent);
 
 document.addEventListener('stylishUpdate', onUpdateClicked);
 document.addEventListener('stylishUpdateChrome', onUpdateClicked);
diff --git a/js/dom.js b/js/dom.js
index 249de722..630d0738 100644
--- a/js/dom.js
+++ b/js/dom.js
@@ -1,6 +1,6 @@
 'use strict';
 
-if (!navigator.userAgent.includes('Windows')) {
+if (!/^Win\d+/.test(navigator.platform)) {
   document.documentElement.classList.add('non-windows');
 }
 
@@ -38,7 +38,7 @@ for (const type of [NodeList, NamedNodeMap, HTMLCollection, HTMLAllCollection])
 
 onDOMready().then(() => $('#firefox-transitions-bug-suppressor').remove());
 
-if (navigator.userAgent.includes('Firefox')) {
+if (!chrome.app) {
   // die if unable to access BG directly
   chrome.windows.getCurrent(wnd => {
     if (!BG && wnd.incognito) {
diff --git a/js/messaging.js b/js/messaging.js
index 6d4a19a7..164eafc7 100644
--- a/js/messaging.js
+++ b/js/messaging.js
@@ -1,11 +1,22 @@
 /* global BG: true, onRuntimeMessage, applyOnMessage, handleUpdate, handleDelete */
+/* global FIREFOX: true */
 'use strict';
 
 // keep message channel open for sendResponse in chrome.runtime.onMessage listener
 const KEEP_CHANNEL_OPEN = true;
 
-const FIREFOX = parseFloat(navigator.userAgent.match(/\bFirefox\/(\d+\.\d+)|$/)[1]);
-const OPERA = parseFloat(navigator.userAgent.match(/\bOPR\/(\d+\.\d+)|$/)[1]);
+let FIREFOX = !chrome.app && parseFloat(navigator.userAgent.match(/\bFirefox\/(\d+\.\d+)|$/)[1]);
+const OPERA = chrome.app && parseFloat(navigator.userAgent.match(/\bOPR\/(\d+\.\d+)|$/)[1]);
+
+if (!chrome.app && !chrome.browserAction.openPopup) {
+  // in FF pre-57 legacy addons can override useragent so we assume the worst
+  // until we know for sure in the async getBrowserInfo()
+  // (browserAction.openPopup was added in 57)
+  FIREFOX = 50;
+  browser.runtime.getBrowserInfo().then(info => {
+    FIREFOX = parseFloat(info.version);
+  });
+}
 
 const URLS = {
   ownOrigin: chrome.runtime.getURL(''),
@@ -29,6 +40,7 @@ const URLS = {
   // Chrome 61.0.3161+ doesn't run content scripts on NTP https://crrev.com/2978953002/
   // TODO: remove when "minimum_chrome_version": "61" or higher
   chromeProtectsNTP:
+    chrome.app &&
     parseInt(navigator.userAgent.match(/Chrom\w+\/(?:\d+\.){2}(\d+)|$/)[1]) >= 3161,
 
   supported: url => (
@@ -52,7 +64,7 @@ if (!BG || BG !== window) {
   document.documentElement.classList.toggle('opera', OPERA);
   // TODO: remove once our manifest's minimum_chrome_version is 50+
   // Chrome 49 doesn't report own extension pages in webNavigation apparently
-  if (navigator.userAgent.includes('Chrome/49.')) {
+  if (chrome.app && navigator.userAgent.includes('Chrome/49.')) {
     getActiveTab().then(BG.updateIcon);
   }
 }
diff --git a/js/prefs.js b/js/prefs.js
index 7938a475..70adcefb 100644
--- a/js/prefs.js
+++ b/js/prefs.js
@@ -317,7 +317,7 @@ var prefs = new function Prefs() {
   }
 
   function contextDeleteMissing() {
-    return (
+    return chrome.app && (
       // detect browsers without Delete by looking at the end of UA string
       /Vivaldi\/[\d.]+$/.test(navigator.userAgent) ||
       // Chrome and co.