--- a/toolkit/components/extensions/ExtensionCommon.jsm
+++ b/toolkit/components/extensions/ExtensionCommon.jsm
@@ -152,31 +152,50 @@ class BaseContext {
 
     MessageChannel.setupMessageManagers([this.messageManager]);
 
     let isValidInnerWindow = () => {
       return !Cu.isDeadWrapper(contentWindow) &&
         getInnerWindowID(contentWindow) === this.innerWindowID;
     };
 
+    function defineWritableGetter(object, prop, getter) {
+      Object.defineProperty(object, prop, {
+        configurable: true,
+        enumerable: true,
+        get: getter,
+        set(value) {
+          Object.defineProperty(this, prop, {
+            configurable: true,
+            enumerable: true,
+            writable: true,
+            value,
+          });
+        },
+      });
+    }
+
     // Use an event target object (instead of a function) so that the closure
     // can be released upon closing this context, even if the document is in the
     // bfcache.
     let handler = {
       handleEvent: (event) => {
         if (event.target !== document) {
           return;
         }
         if (event.type === "pageshow") {
           this.contentWindow = contentWindow;
           this.active = true;
         }
         if (event.type === "pagehide") {
-          this.contentWindow = null;
-          this.active = false;
+          // Define getters instead of properties, so that the "contentWindow"
+          // and "active" properties are accurate during the "pageshow" event
+          // and during "unload".
+          defineWritableGetter(this, "contentWindow", () => isValidInnerWindow() ? contentWindow : null);
+          defineWritableGetter(this, "active", isValidInnerWindow);
         }
       },
     };
 
     this.contentWindow = contentWindow;
     this.active = true;
     contentWindow.addEventListener("pagehide", handler, {mozSystemGroup: true}, false);
     contentWindow.addEventListener("pageshow", handler, {mozSystemGroup: true}, false);

--- a/toolkit/components/extensions/test/xpcshell/test_ext_contentscript_context.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_contentscript_context.js
@@ -124,8 +124,86 @@ add_task(async function test_contentscri
 
     await this.pageshownPromise;
 
     Assert.equal(this.context.contentWindow, null, "Context's contentWindow property is null after restore from bfcache");
   });
 
   await contentPage.close();
 });
+
+add_task(async function test_contentscript_context_valid_during_execution() {
+  // This test does the following:
+  // - Load page
+  // - Load extension; inject content script.
+  // - Navigate page; pagehide triggered.
+  // - Navigate back; pageshow triggered.
+  // - Close page; pagehide, unload triggered.
+  // At each of these last four events, the validity of the context is checked.
+
+  function contentScript() {
+    browser.test.sendMessage("content-script-ready");
+    window.wrappedJSObject.checkContextIsValid("Context is valid on execution");
+
+    window.addEventListener("pagehide", () => {
+      window.wrappedJSObject.checkContextIsValid("Context is valid on pagehide");
+      browser.test.sendMessage("content-script-hide");
+    }, true);
+    window.addEventListener("pageshow", () => {
+      window.wrappedJSObject.checkContextIsValid("Context is valid on pageshow");
+
+      // This unload listener is registered after pageshow, to ensure that the
+      // page can be stored in the bfcache at the previous pagehide.
+      window.addEventListener("unload", () => {
+        window.wrappedJSObject.checkContextIsValid("Context is valid on unload");
+        browser.test.sendMessage("content-script-unload");
+      });
+
+      browser.test.sendMessage("content-script-show");
+    });
+  }
+
+  let extension = ExtensionTestUtils.loadExtension({
+    manifest: {
+      content_scripts: [{
+        "matches": ["http://example.com/dummy"],
+        "js": ["content_script.js"],
+      }],
+    },
+
+    files: {
+      "content_script.js": contentScript,
+    },
+  });
+
+  let contentPage = await ExtensionTestUtils.loadContentPage("http://example.com/dummy");
+  await contentPage.spawn(extension.id, async extensionId => {
+    let context;
+    let checkContextIsValid = (description) => {
+      if (!context) {
+        let {DocumentManager} = ChromeUtils.import("resource://gre/modules/ExtensionContent.jsm", {});
+        context = DocumentManager.getContext(extensionId, this.content);
+      }
+      Assert.equal(context.contentWindow, this.content, `${description}: contentWindow`);
+      Assert.equal(context.active, true, `${description}: contentWindow`);
+    };
+    Cu.exportFunction(checkContextIsValid, this.content, {defineAs: "checkContextIsValid"});
+  });
+  await extension.startup();
+  await extension.awaitMessage("content-script-ready");
+
+  await contentPage.spawn(extension.id, async extensionId => {
+    // Navigate so that the content page is hidden in the bfcache.
+    this.content.location = "http://example.org/dummy";
+  });
+
+  await extension.awaitMessage("content-script-hide");
+  await contentPage.spawn(null, async () => {
+    // Navigate back so the content page is resurrected from the bfcache.
+    this.content.history.back();
+  });
+
+  await extension.awaitMessage("content-script-show");
+  await contentPage.close();
+  await extension.awaitMessage("content-script-hide");
+  await extension.awaitMessage("content-script-unload");
+  await extension.unload();
+});

--- a/toolkit/components/extensions/test/xpcshell/test_ext_contentscript_context_isolation.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_contentscript_context_isolation.js
@@ -79,33 +79,39 @@ add_task(async function test_contentscri
 
     this.content.location = "http://example.org/dummy";
   });
 
   await extension.awaitMessage("content-script-hide");
 
   await contentPage.spawn(null, async () => {
     Assert.equal(this.context.contentWindow, null, "Context's contentWindow property is null");
+    Assert.ok(this.context.sandbox, "Context's sandbox exists");
 
     // Navigate back so the content page is resurrected from the bfcache.
     this.content.history.back();
   });
 
   await extension.awaitMessage("content-script-show");
 
   await contentPage.spawn(null, async () => {
     Assert.equal(this.context.contentWindow, this.content, "Context's contentWindow property is correct");
+    Assert.ok(this.context.sandbox, "Context's sandbox exists before unload");
 
     // Now add an "unload" event listener, which should prevent a page from entering the bfcache.
     await new Promise((resolve) => {
       this.content.addEventListener("unload", () => {
-        Assert.equal(this.context.contentWindow, null, "Context's contentWindow property is null");
+        Assert.equal(this.context.contentWindow, this.content, "Context's contentWindow property is null");
         resolve();
       });
       this.content.location = "http://example.org/dummy";
     });
   });
 
   await extension.awaitMessage("content-script-unload");
 
+  await contentPage.spawn(null, async () => {
+    Assert.equal(this.context.sandbox, null, "Context's sandbox has been destroyed after unload");
+  });
+
   await contentPage.close();
   await extension.unload();
 });