--- a/toolkit/components/extensions/ext-downloads.js
+++ b/toolkit/components/extensions/ext-downloads.js
@@ -41,16 +41,18 @@ const DOWNLOAD_ITEM_CHANGE_FIELDS = ["en
 const FORBIDDEN_HEADERS = ["ACCEPT-CHARSET", "ACCEPT-ENCODING",
                            "ACCESS-CONTROL-REQUEST-HEADERS", "ACCESS-CONTROL-REQUEST-METHOD",
                            "CONNECTION", "CONTENT-LENGTH", "COOKIE", "COOKIE2", "DATE", "DNT",
                            "EXPECT", "HOST", "KEEP-ALIVE", "ORIGIN", "REFERER", "TE", "TRAILER",
                            "TRANSFER-ENCODING", "UPGRADE", "VIA"];
 
 const FORBIDDEN_PREFIXES = /^PROXY-|^SEC-/i;
 
+const PROMPTLESS_DOWNLOAD_PREF = "browser.download.useDownloadDir";
+
 class DownloadItem {
   constructor(id, download, extension) {
     this.id = id;
     this.download = download;
     this.extension = extension;
     this.prechange = {};
   }
 
@@ -476,42 +478,53 @@ this.downloads = class extends Extension
               let uri = Services.io.newURI(options.url);
               if (uri instanceof Ci.nsIURL) {
                 filename = DownloadPaths.sanitize(uri.fileName);
               }
             }
 
             let target = OS.Path.join(downloadsDir, filename || "download");
 
+            let saveAs;
+            if (options.saveAs !== null) {
+              saveAs = options.saveAs;
+            } else {
+              // If options.saveAs was not specified, only show the file chooser
+              // if |browser.download.useDownloadDir == false|. That is to say,
+              // only show the file chooser if Firefox normally shows it when
+              // a file is downloaded.
+              saveAs = !Services.prefs.getBoolPref(PROMPTLESS_DOWNLOAD_PREF, true);
+            }
+
             // Create any needed subdirectories if required by filename.
             const dir = OS.Path.dirname(target);
             await OS.File.makeDir(dir, {from: downloadsDir});
 
             if (await OS.File.exists(target)) {
               // This has a race, something else could come along and create
               // the file between this test and them time the download code
               // creates the target file.  But we can't easily fix it without
               // modifying DownloadCore so we live with it for now.
               switch (options.conflictAction) {
                 case "uniquify":
                 default:
                   target = DownloadPaths.createNiceUniqueFile(new FileUtils.File(target)).path;
-                  if (options.saveAs) {
+                  if (saveAs) {
                     // createNiceUniqueFile actually creates the file, which
                     // is premature if we need to show a SaveAs dialog.
                     await OS.File.remove(target);
                   }
                   break;
 
                 case "overwrite":
                   break;
               }
             }
 
-            if (!options.saveAs) {
+            if (!saveAs) {
               return target;
             }
 
             // Setup the file picker Save As dialog.
             const picker = Cc["@mozilla.org/filepicker;1"].createInstance(Ci.nsIFilePicker);
             const window = Services.wm.getMostRecentWindow("navigator:browser");
             picker.init(window, null, Ci.nsIFilePicker.modeSave);
             picker.displayDirectory = new FileUtils.File(dir);

--- a/toolkit/components/extensions/schemas/downloads.json
+++ b/toolkit/components/extensions/schemas/downloads.json
@@ -382,17 +382,17 @@
                 "default": false,
                 "type": "boolean"
               },
               "conflictAction": {
                 "$ref": "FilenameConflictAction",
                 "optional": true
               },
               "saveAs": {
-                "description": "Use a file-chooser to allow the user to select a filename.",
+                "description": "Use a file-chooser to allow the user to select a filename. If the option is not specified, the file chooser will be shown only if the Firefox \"Always ask you where to save files\" option is enabled (i.e. the pref <code>browser.download.useDownloadDir</code> is set to <code>false</code>).",
                 "optional": true,
                 "type": "boolean"
               },
               "method": {
                 "description": "The HTTP method to use if the URL uses the HTTP[S] protocol.",
                 "enum": [
                   "GET",
                   "POST"

--- a/toolkit/components/extensions/test/mochitest/chrome.ini
+++ b/toolkit/components/extensions/test/mochitest/chrome.ini
@@ -15,16 +15,17 @@ tags = webextensions in-process-webexten
 
 [test_chrome_ext_background_page.html]
 skip-if = (toolkit == 'android') # android doesn't have devtools
 [test_chrome_ext_contentscript_data_uri.html]
 [test_chrome_ext_contentscript_telemetry.html]
 [test_chrome_ext_contentscript_unrecognizedprop_warning.html]
 [test_chrome_ext_downloads_open.html]
 [test_chrome_ext_downloads_saveAs.html]
+[test_chrome_ext_downloads_uniquify.html]
 [test_chrome_ext_eventpage_warning.html]
 [test_chrome_ext_idle.html]
 [test_chrome_ext_identity.html]
 skip-if = os == 'android' # unsupported.
 [test_chrome_ext_permissions.html]
 skip-if = os == 'android' # Bug 1350559
 [test_chrome_ext_storage_cleanup.html]
 [test_chrome_ext_trackingprotection.html]

--- a/toolkit/components/extensions/test/mochitest/test_chrome_ext_downloads_saveAs.html
+++ b/toolkit/components/extensions/test/mochitest/test_chrome_ext_downloads_saveAs.html
@@ -11,60 +11,78 @@
 </head>
 <body>
 
 <script type="text/javascript">
 "use strict";
 
 Cu.import("resource://gre/modules/FileUtils.jsm");
 
-let directory;
+const PROMPTLESS_DOWNLOAD_PREF = "browser.download.useDownloadDir";
+
+// We need to be able to distinguish files downloaded by the file picker from
+// files downloaded without it.
+let pickerDir;
+let defaultDir;
 
 add_task(async function setup() {
-  directory = FileUtils.getDir("TmpD", ["downloads"]);
-  directory.createUnique(Ci.nsIFile.DIRECTORY_TYPE, FileUtils.PERMS_DIRECTORY);
-  info(`Using download directory ${directory.path}`);
+  let downloadDir = FileUtils.getDir("TmpD", ["downloads"]);
+  pickerDir = downloadDir.clone();
+  pickerDir.createUnique(Ci.nsIFile.DIRECTORY_TYPE, FileUtils.PERMS_DIRECTORY);
+  info(`Using file picker download directory ${pickerDir.path}`);
+  defaultDir = downloadDir.clone();
+  defaultDir.createUnique(Ci.nsIFile.DIRECTORY_TYPE, FileUtils.PERMS_DIRECTORY);
+  info(`Using default download directory ${defaultDir.path}`);
+
+  isnot(pickerDir.path, defaultDir.path,
+        "Should be able to distinguish between files saved with or without the file picker");
 
   await SpecialPowers.pushPrefEnv({"set": [
     ["browser.download.folderList", 2],
-    ["browser.download.dir", directory.path],
+    ["browser.download.dir", defaultDir.path],
   ]});
 
   SimpleTest.registerCleanupFunction(async () => {
     await SpecialPowers.popPrefEnv();
-    directory.remove(true);
+    pickerDir.remove(true);
+    defaultDir.remove(true);
   });
 });
 
 add_task(async function test_downloads_saveAs() {
-  const file = directory.clone();
-  file.append("file_download.txt");
+  const pickerFile = pickerDir.clone();
+  pickerFile.append("file_download.txt");
 
-  const unique = directory.clone();
-  unique.append("file_download(1).txt");
+  const defaultFile = defaultDir.clone();
+  defaultFile.append("file_download.txt");
 
   const {MockFilePicker} = SpecialPowers;
   MockFilePicker.init(window);
 
   MockFilePicker.showCallback = fp => {
-    let file = directory.clone();
+    // On picker 'show' event, choose the filename that was set as the default
+    // and append it to the picker's download directory
+    let file = pickerDir.clone();
     file.append(fp.defaultString);
     MockFilePicker.setFiles([file]);
   };
 
   function background() {
     const url = URL.createObjectURL(new Blob(["file content"]));
-    browser.test.onMessage.addListener(async filename => {
+    browser.test.onMessage.addListener(async (filename, saveAs) => {
       try {
-        let id = await browser.downloads.download({
+        let options = {
           url,
           filename,
-          saveAs: true,
-          conflictAction: "uniquify",
-        });
+        };
+        // Only define the saveAs option if the argument was actually set
+        if (saveAs !== undefined) {
+          options.saveAs = saveAs;
+        }
+        let id = await browser.downloads.download(options);
         browser.downloads.onChanged.addListener(delta => {
           if (delta.id == id && delta.state.current === "complete") {
             browser.test.sendMessage("done", {ok: true, id});
           }
         });
       } catch ({message}) {
         browser.test.sendMessage("done", {ok: false, message});
       }
@@ -73,58 +91,78 @@ add_task(async function test_downloads_s
   }
 
   const manifest = {background, manifest: {permissions: ["downloads"]}};
   const extension = ExtensionTestUtils.loadExtension(manifest);
 
   await extension.startup();
   await extension.awaitMessage("ready");
 
-  // Test basic saveAs functionality.
-  MockFilePicker.returnValue = MockFilePicker.returnOK;
+  async function testExpectFilePicker(saveAs) {
+    ok(!pickerFile.exists(), "the file should have been cleaned up properly previously");
+
+    MockFilePicker.returnValue = MockFilePicker.returnOK;
 
-  extension.sendMessage("file_download.txt");
-  let result = await extension.awaitMessage("done");
-  ok(result.ok, "downloads.download() works with saveAs");
+    extension.sendMessage("file_download.txt", saveAs);
+    let result = await extension.awaitMessage("done");
+    ok(result.ok, `downloads.download() works with saveAs=${saveAs}`);
 
-  ok(file.exists(), "the file exists.");
-  is(file.fileSize, 12, "downloaded file is the correct size");
+    ok(pickerFile.exists(), "the file exists.");
+    is(pickerFile.fileSize, 12, "downloaded file is the correct size");
+    pickerFile.remove(false);
 
-  // Test exisisting file with uniquify.
-  MockFilePicker.returnValue = MockFilePicker.returnOK;
+    // Test the user canceling the save dialog.
+    MockFilePicker.returnValue = MockFilePicker.returnCancel;
+
+    extension.sendMessage("file_download.txt", saveAs);
+    result = await extension.awaitMessage("done");
 
-  extension.sendMessage("file_download.txt");
-  result = await extension.awaitMessage("done");
-  ok(result.ok, "downloads.download() works with saveAs and uniquify");
+    ok(!result.ok, "download rejected if the user cancels the dialog");
+    is(result.message, "Download canceled by the user", "with the correct message");
+    ok(!pickerFile.exists(), "file was not downloaded");
+  }
 
-  ok(unique.exists(), "the file exists.");
-  is(unique.fileSize, 12, "downloaded file is the correct size");
-  unique.remove(false);
+  async function testNoFilePicker(saveAs) {
+    ok(!defaultFile.exists(), "the file should have been cleaned up properly previously");
+
+    extension.sendMessage("file_download.txt", saveAs);
+    let result = await extension.awaitMessage("done");
+    ok(result.ok, `downloads.download() works with saveAs=${saveAs}`);
 
-  // Test canceled saveAs for an existing file and uniquify.
-  MockFilePicker.returnValue = MockFilePicker.returnCancel;
+    ok(defaultFile.exists(), "the file exists.");
+    is(defaultFile.fileSize, 12, "downloaded file is the correct size");
+    defaultFile.remove(false);
+  }
 
-  extension.sendMessage("file_download.txt");
-  result = await extension.awaitMessage("done");
+  info("Testing that saveAs=true uses the file picker as expected");
+  let saveAs = true;
+  await testExpectFilePicker(saveAs);
 
-  ok(!result.ok, "download rejected if the user cancels the dialog");
-  is(result.message, "Download canceled by the user", "with the correct message");
+  info("Testing that saveAs=false does not use the file picker");
+  saveAs = false;
+  await testNoFilePicker(saveAs);
 
-  ok(!unique.exists(), "unique file not left after SaveAs canceled.");
-  file.remove(false);
-
-  // Test the user canceling the save dialog.
-  MockFilePicker.returnValue = MockFilePicker.returnCancel;
+  // When saveAs is not set, the behavior should be determined by the Firefox
+  // pref that normally determines whether the "Save As" prompt should be
+  // displayed.
+  info(`Testing that the file picker is used when saveAs is not specified ` +
+       `but ${PROMPTLESS_DOWNLOAD_PREF} is disabled`);
+  saveAs = undefined;
+  await SpecialPowers.pushPrefEnv({"set": [
+    [PROMPTLESS_DOWNLOAD_PREF, false],
+  ]});
+  await testExpectFilePicker(saveAs);
 
-  extension.sendMessage("file_download.txt");
-  result = await extension.awaitMessage("done");
-
-  ok(!result.ok, "download rejected if the user cancels the dialog");
-  is(result.message, "Download canceled by the user", "with the correct message");
-  ok(!file.exists(), "file was not downloaded");
+  info(`Testing that the file picker is NOT used when saveAs is not ` +
+       `specified but ${PROMPTLESS_DOWNLOAD_PREF} is enabled`);
+  await SpecialPowers.popPrefEnv();
+  await SpecialPowers.pushPrefEnv({"set": [
+    [PROMPTLESS_DOWNLOAD_PREF, true],
+  ]});
+  await testNoFilePicker(saveAs);
 
   await extension.unload();
   MockFilePicker.cleanup();
 });
 
 </script>
 
 </body>

copy from toolkit/components/extensions/test/mochitest/test_chrome_ext_downloads_saveAs.html
copy to toolkit/components/extensions/test/mochitest/test_chrome_ext_downloads_uniquify.html
--- a/toolkit/components/extensions/test/mochitest/test_chrome_ext_downloads_saveAs.html
+++ b/toolkit/components/extensions/test/mochitest/test_chrome_ext_downloads_uniquify.html
@@ -1,12 +1,12 @@
 <!doctype html>
 <html>
 <head>
-  <title>Test downloads.download() saveAs option</title>
+  <title>Test downloads.download() uniquify option</title>
   <script src="chrome://mochikit/content/tests/SimpleTest/SpawnTask.js"></script>
   <script src="chrome://mochikit/content/tests/SimpleTest/SimpleTest.js"></script>
   <script src="chrome://mochikit/content/tests/SimpleTest/ExtensionTestUtils.js"></script>
   <script src="chrome_head.js"></script>
   <script src="head.js"></script>
   <link rel="stylesheet" href="chrome://mochikit/content/tests/SimpleTest/test.css"/>
 </head>
 <body>
@@ -29,40 +29,41 @@ add_task(async function setup() {
   ]});
 
   SimpleTest.registerCleanupFunction(async () => {
     await SpecialPowers.popPrefEnv();
     directory.remove(true);
   });
 });
 
-add_task(async function test_downloads_saveAs() {
+add_task(async function test_downloads_uniquify() {
   const file = directory.clone();
   file.append("file_download.txt");
 
   const unique = directory.clone();
   unique.append("file_download(1).txt");
 
   const {MockFilePicker} = SpecialPowers;
   MockFilePicker.init(window);
+  MockFilePicker.returnValue = MockFilePicker.returnOK;
 
   MockFilePicker.showCallback = fp => {
     let file = directory.clone();
     file.append(fp.defaultString);
     MockFilePicker.setFiles([file]);
   };
 
   function background() {
     const url = URL.createObjectURL(new Blob(["file content"]));
-    browser.test.onMessage.addListener(async filename => {
+    browser.test.onMessage.addListener(async (filename, saveAs) => {
       try {
         let id = await browser.downloads.download({
           url,
           filename,
-          saveAs: true,
+          saveAs,
           conflictAction: "uniquify",
         });
         browser.downloads.onChanged.addListener(delta => {
           if (delta.id == id && delta.state.current === "complete") {
             browser.test.sendMessage("done", {ok: true, id});
           }
         });
       } catch ({message}) {
@@ -73,58 +74,44 @@ add_task(async function test_downloads_s
   }
 
   const manifest = {background, manifest: {permissions: ["downloads"]}};
   const extension = ExtensionTestUtils.loadExtension(manifest);
 
   await extension.startup();
   await extension.awaitMessage("ready");
 
-  // Test basic saveAs functionality.
-  MockFilePicker.returnValue = MockFilePicker.returnOK;
+  async function testUniquify(saveAs) {
+    info(`Testing conflictAction:"uniquify" with saveAs=${saveAs}`);
 
-  extension.sendMessage("file_download.txt");
-  let result = await extension.awaitMessage("done");
-  ok(result.ok, "downloads.download() works with saveAs");
-
-  ok(file.exists(), "the file exists.");
-  is(file.fileSize, 12, "downloaded file is the correct size");
+    ok(!file.exists(), "downloaded file should have been cleaned up before test ran");
+    ok(!unique.exists(), "uniquified file should have been cleaned up before test ran");
 
-  // Test exisisting file with uniquify.
-  MockFilePicker.returnValue = MockFilePicker.returnOK;
-
-  extension.sendMessage("file_download.txt");
-  result = await extension.awaitMessage("done");
-  ok(result.ok, "downloads.download() works with saveAs and uniquify");
-
-  ok(unique.exists(), "the file exists.");
-  is(unique.fileSize, 12, "downloaded file is the correct size");
-  unique.remove(false);
+    // Test download without uniquify and create a conflicting file so we can
+    // test with uniquify.
+    extension.sendMessage("file_download.txt", saveAs);
+    let result = await extension.awaitMessage("done");
+    ok(result.ok, "downloads.download() works with saveAs");
 
-  // Test canceled saveAs for an existing file and uniquify.
-  MockFilePicker.returnValue = MockFilePicker.returnCancel;
+    ok(file.exists(), "the file exists.");
+    is(file.fileSize, 12, "downloaded file is the correct size");
 
-  extension.sendMessage("file_download.txt");
-  result = await extension.awaitMessage("done");
-
-  ok(!result.ok, "download rejected if the user cancels the dialog");
-  is(result.message, "Download canceled by the user", "with the correct message");
+    // Now that a conflicting file exists, test the uniquify behavior
+    extension.sendMessage("file_download.txt", saveAs);
+    result = await extension.awaitMessage("done");
+    ok(result.ok, "downloads.download() works with saveAs and uniquify");
 
-  ok(!unique.exists(), "unique file not left after SaveAs canceled.");
-  file.remove(false);
-
-  // Test the user canceling the save dialog.
-  MockFilePicker.returnValue = MockFilePicker.returnCancel;
+    ok(unique.exists(), "the file exists.");
+    is(unique.fileSize, 12, "downloaded file is the correct size");
 
-  extension.sendMessage("file_download.txt");
-  result = await extension.awaitMessage("done");
-
-  ok(!result.ok, "download rejected if the user cancels the dialog");
-  is(result.message, "Download canceled by the user", "with the correct message");
-  ok(!file.exists(), "file was not downloaded");
+    file.remove(false);
+    unique.remove(false);
+  }
+  await testUniquify(true);
+  await testUniquify(false);
 
   await extension.unload();
   MockFilePicker.cleanup();
 });
 
 </script>
 
 </body>