diff --git a/chrome/content/zotero/xpcom/storage/webdav.js b/chrome/content/zotero/xpcom/storage/webdav.js
index 1633efc1c8..b6953d14c7 100644
--- a/chrome/content/zotero/xpcom/storage/webdav.js
+++ b/chrome/content/zotero/xpcom/storage/webdav.js
@@ -521,6 +521,10 @@ Zotero.Sync.Storage.Mode.WebDAV.prototype = {
 		file.append(item.key + '.zip');
 		Components.utils.importGlobalProperties(["File"]);
 		file = File.createFromFileName ? File.createFromFileName(file.path) : new File(file);
+		// File.createFromFileName() returns a Promise in Fx54+
+		if (file.then) {
+			file = yield file;
+		}
 		
 		var uri = this._getItemURI(item);
 		
diff --git a/chrome/content/zotero/xpcom/storage/zfs.js b/chrome/content/zotero/xpcom/storage/zfs.js
index 15d3af8be8..563a78a3c6 100644
--- a/chrome/content/zotero/xpcom/storage/zfs.js
+++ b/chrome/content/zotero/xpcom/storage/zfs.js
@@ -613,6 +613,10 @@ Zotero.Sync.Storage.Mode.ZFS.prototype = {
 		
 		Components.utils.importGlobalProperties(["File"]);
 		file = File.createFromFileName ? File.createFromFileName(file.path) : new File(file);
+		// File.createFromFileName() returns a Promise in Fx54+
+		if (file.then) {
+			file = yield file;
+		}
 		
 		var blob = new Blob([params.prefix, file, params.suffix]);
 		
diff --git a/test/tests/zfsTest.js b/test/tests/zfsTest.js
index 434bad715e..f21bd3624b 100644
--- a/test/tests/zfsTest.js
+++ b/test/tests/zfsTest.js
@@ -275,6 +275,11 @@ describe("Zotero.Sync.Storage.Mode.ZFS", function () {
 			var suffix1 = Zotero.Utilities.randomString();
 			var uploadKey1 = Zotero.Utilities.randomString(32, 'abcdef0123456789');
 			
+			let file1Blob = File.createFromFileName ? File.createFromFileName(file1.path) : new File(file1);
+			if (file1Blob.then) {
+				file1Blob = yield file1Blob;
+			}
+			
 			// HTML file with auxiliary image
 			var file2 = OS.Path.join(getTestDataDirectory().path, 'snapshot', 'index.html');
 			var parentItem = yield createDataObject('item');
@@ -397,9 +402,7 @@ describe("Zotero.Sync.Storage.Mode.ZFS", function () {
 						(new Blob(
 							[
 								prefix1,
-								File.createFromFileName
-									? File.createFromFileName(file1.path)
-									: new File(file1),
+								file1Blob,
 								suffix1
 							]
 						).size)