diff --git a/_locales/en/messages.json b/_locales/en/messages.json
index 1f0e84cf..57d078bf 100644
--- a/_locales/en/messages.json
+++ b/_locales/en/messages.json
@@ -1031,6 +1031,18 @@
     "message": "Stylus can access file:// URLs only if you enable the corresponding checkbox for Stylus extension on chrome://extensions page.",
     "description": "Note in the toolbar popup for file:// URLs"
   },
+  "unreachableAMO": {
+    "message": "Firefox forbids access to the site.",
+    "description": "Note in the popup displayed when opened on addons.mozilla.org"
+  },
+  "unreachableAMOHint": {
+    "message": "To allow access open <about:config>, right-click the list, click 'New', then 'Boolean', paste <privacy.resistFingerprinting.block_mozAddonManager> and click OK, <true>, OK, reload the <addons.mozilla.org> page.",
+    "description": "Note in the popup when opened on addons.mozilla.org in Firefox >= 59"
+  },
+  "unreachableAMOHintOldFF": {
+    "message": "Only Firefox 59 and newer can be configured to allow WebExtensions to add style elements on CSP-protected sites such as this one.",
+    "description": "Note in the popup when opened on addons.mozilla.org in Firefox < 59"
+  },
   "updateCheckFailBadResponseCode": {
     "message": "Update failed: server responded with code $code$.",
     "description": "Text that displays when an update check failed because the response code indicates an error",
diff --git a/js/messaging.js b/js/messaging.js
index c4abfe81..fab5d692 100644
--- a/js/messaging.js
+++ b/js/messaging.js
@@ -45,7 +45,7 @@ const URLS = {
   chromeProtectsNTP: CHROME >= 3161,
 
   supported: url => (
-    url.startsWith('http') && !url.startsWith(URLS.browserWebStore) ||
+    url.startsWith('http') && (FIREFOX || !url.startsWith(URLS.browserWebStore)) ||
     url.startsWith('ftp') ||
     url.startsWith('file') ||
     url.startsWith(URLS.ownOrigin) ||
diff --git a/popup/popup.js b/popup/popup.js
index d328cf1e..f9e6fb8d 100644
--- a/popup/popup.js
+++ b/popup/popup.js
@@ -122,17 +122,24 @@ function initPopup(url) {
       ignoreChromeError();
       // FF and some Chrome forks (e.g. CentBrowser) implement tab-on-demand
       // so we'll wait a bit to handle popup being invoked right after switching
-      if (
-        retryCountdown > 0 && (
+      if (retryCountdown > 0 && (
           tab.status !== 'complete' ||
-          FIREFOX && tab.url === 'about:blank'
-        )
-      ) {
+          FIREFOX && tab.url === 'about:blank')) {
         setTimeout(ping, 100, tab, --retryCountdown);
-      } else {
-        document.body.classList.add('unreachable');
-        document.body.insertBefore(template.unreachableInfo, document.body.firstChild);
+        return;
       }
+      const info = template.unreachableInfo;
+      if (FIREFOX && tabURL.startsWith(URLS.browserWebStore)) {
+        $('label', info).textContent = t('unreachableAMO');
+        $('p', info).insertAdjacentElement('afterend',
+          $create('p',
+            t(FIREFOX < 59 ? 'unreachableAMOHintOldFF' : 'unreachableAMOHint')
+              .split(/(<.*?>)/)
+              .map(s => s[0] === '<' ? $create('b', s.slice(1, -1)) : s)));
+        $('p', info).remove();
+      }
+      document.body.classList.add('unreachable');
+      document.body.insertBefore(info, document.body.firstChild);
     });
   });