disconnect port explicitly in FF

content/install-hook-usercss.js

@@ -15,6 +15,8 @@ if (typeof self.oldCode !== 'string') {
           if (msg.code != null) self.oldCode = msg.code;
         });
     });
+    // FF keeps content scripts connected on navigation https://github.com/openstyles/stylus/issues/864
+    addEventListener('pagehide', () => port.disconnect(), {once: true});
   });
 }
 

install-usercss/install-usercss.js

@@ -398,9 +398,10 @@
         }
       });
       port.onDisconnect.addListener(() => {
-        browser.tabs.get(tabId)
-          .then(tab => tab.url === initialUrl && location.reload())
-          .catch(closeCurrentTab);
+        chrome.tabs.get(tabId, tab =>
+          !chrome.runtime.lastError && tab.url === initialUrl
+            ? location.reload()
+            : closeCurrentTab());
       });
       return (opts = {}) => new Promise((resolve, reject) => {
         const id = performance.now();