--- a/toolkit/components/printing/content/printUtils.js
+++ b/toolkit/components/printing/content/printUtils.js
@@ -195,18 +195,20 @@ var PrintUtils = {
       this._originalURL = this._sourceBrowser.currentURI.spec;
 
       // Here we log telemetry data for when the user enters print preview.
       this.logTelemetry("PRINT_PREVIEW_OPENED_COUNT");
     } else {
       // collapse the browser here -- it will be shown in
       // enterPrintPreview; this forces a reflow which fixes display
       // issues in bug 267422.
-      let ppBrowser = this._listener.getPrintPreviewBrowser();
-      ppBrowser.collapsed = true;
+      // We use the print preview browser as the source browser to avoid
+      // re-initializing print preview with a document that might now have changed.
+      this._sourceBrowser = this._listener.getPrintPreviewBrowser();
+      this._sourceBrowser.collapsed = true;
 
       // If the user transits too quickly within preview and we have a pending
       // progress dialog, we will close it before opening a new one.
       if (this._webProgressPP && this._webProgressPP.value) {
         this._webProgressPP.value.onStateChange(null, null,
           Components.interfaces.nsIWebProgressListener.STATE_STOP, 0);
       }
     }
@@ -517,17 +519,17 @@ var PrintUtils = {
           spMM.removeMessageListener("Printing:Preview:ReaderModeReady", onReaderReady);
           sendEnterPreviewMessage(simplifiedBrowser, true);
         });
 
         // Here, we send down a message to simplified browser in order to parse
         // the original page. After we have parsed it, content will tell parent
         // that the document is ready for print previewing.
         spMM.sendAsyncMessage("Printing:Preview:ParseDocument", {
-          URL: this._sourceBrowser.currentURI.spec,
+          URL: this._originalURL,
           windowID: this._sourceBrowser.outerWindowID,
         });
 
         // Here we log telemetry data for when the user enters simplify mode.
         this.logTelemetry("PRINT_PREVIEW_SIMPLIFY_PAGE_OPENED_COUNT");
       }
     } else {
       sendEnterPreviewMessage(this._sourceBrowser, false);

--- a/toolkit/components/printing/moz.build
+++ b/toolkit/components/printing/moz.build
@@ -1,10 +1,14 @@
 # -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
 # vim: set filetype=python:
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 JAR_MANIFESTS += ['jar.mn']
 
+BROWSER_CHROME_MANIFESTS += [
+    'tests/browser.ini'
+]
+
 with Files('**'):
     BUG_COMPONENT = ('Toolkit', 'Printing')

new file mode 100644
--- /dev/null
+++ b/toolkit/components/printing/tests/browser.ini
@@ -0,0 +1,2 @@
+[browser_page_change_print_original.js]
+skip-if = os == "mac"

new file mode 100644
--- /dev/null
+++ b/toolkit/components/printing/tests/browser_page_change_print_original.js
@@ -0,0 +1,57 @@
+/**
+ * Verify that if the page contents change after print preview is initialized,
+ * and we re-initialize print preview (e.g. by changing page orientation),
+ * we still show (and will therefore print) the original contents.
+ */
+add_task(function* pp_after_orientation_change() {
+  const DATA_URI = `data:text/html,<script>window.onafterprint = function() { setTimeout("window.location = 'data:text/plain,REPLACED PAGE!'", 0); }</script><pre>INITIAL PAGE</pre>`;
+  // Can only do something if we have a print preview UI:
+  if (AppConstants.platform != "win" && AppConstants.platform != "linux") {
+    ok(true, "Can't test if there's no print preview.");
+    return;
+  }
+
+  // Ensure we get a browserStopped for this browser
+  let tab = yield BrowserTestUtils.openNewForegroundTab(gBrowser, DATA_URI, false, true);
+  let browserToPrint = tab.linkedBrowser;
+  let ppBrowser = PrintPreviewListener.getPrintPreviewBrowser();
+
+  // Get a promise now that resolves when the original tab's location changes.
+  let originalTabNavigated = BrowserTestUtils.browserStopped(browserToPrint);
+
+  // Enter print preview:
+  let printPreviewEntered = BrowserTestUtils.waitForMessage(ppBrowser.messageManager, "Printing:Preview:Entered");
+  document.getElementById("cmd_printPreview").doCommand();
+  yield printPreviewEntered;
+
+  // Assert that we are showing the original page
+  yield ContentTask.spawn(ppBrowser, null, function* () {
+    is(content.document.body.textContent, "INITIAL PAGE", "Should have initial page print previewed.");
+  });
+
+  yield originalTabNavigated;
+
+  // Change orientation and wait for print preview to re-enter:
+  let orient = PrintUtils.getPrintSettings().orientation;
+  let orientToSwitchTo = orient != Ci.nsIPrintSettings.kPortraitOrientation ?
+    "portrait" : "landscape";
+  let printPreviewToolbar = document.querySelector("toolbar[printpreview=true]");
+
+  printPreviewEntered = BrowserTestUtils.waitForMessage(ppBrowser.messageManager, "Printing:Preview:Entered");
+  printPreviewToolbar.orient(orientToSwitchTo);
+  yield printPreviewEntered;
+
+  // Check that we're still showing the original page.
+  yield ContentTask.spawn(ppBrowser, null, function* () {
+    is(content.document.body.textContent, "INITIAL PAGE", "Should still have initial page print previewed.");
+  });
+
+  // Check that the other tab is definitely showing the new page:
+  yield ContentTask.spawn(browserToPrint, null, function* () {
+    is(content.document.body.textContent, "REPLACED PAGE!", "Original page should have changed.");
+  });
+
+  PrintUtils.exitPrintPreview();
+
+  yield BrowserTestUtils.removeTab(tab);
+});