new file mode 100644
--- /dev/null
+++ b/widget/xremoteclient/RemoteUtils.cpp
@@ -0,0 +1,111 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:expandtab:shiftwidth=2:tabstop=8:
+ */
+/* vim:set ts=8 sw=2 et cindent: */
+/* 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/. */
+
+#include <stdlib.h>
+#include <string.h>
+#include <strings.h>
+#include <unistd.h>
+#include <limits.h>
+
+#include "RemoteUtils.h"
+
+#ifdef IS_BIG_ENDIAN
+#define TO_LITTLE_ENDIAN32(x) \
+    ((((x) & 0xff000000) >> 24) | (((x) & 0x00ff0000) >> 8) | \
+    (((x) & 0x0000ff00) << 8) | (((x) & 0x000000ff) << 24))
+#else
+#define TO_LITTLE_ENDIAN32(x) (x)
+#endif
+
+#ifndef MAX_PATH
+#ifdef PATH_MAX
+#define MAX_PATH PATH_MAX
+#else
+#define MAX_PATH 1024
+#endif
+#endif
+
+/* like strcpy, but return the char after the final null */
+static char*
+estrcpy(const char* s, char* d)
+{
+  while (*s)
+    *d++ = *s++;
+
+  *d++ = '\0';
+  return d;
+}
+
+/* Construct a command line from given args and desktop startup ID.
+ * Returned buffer must be released by free().
+ */
+char*
+ConstructCommandLine(int32_t argc, char **argv,
+                     const char* aDesktopStartupID,
+                     int *aCommandLineLength)
+{
+  char cwdbuf[MAX_PATH];
+  if (!getcwd(cwdbuf, MAX_PATH))
+    return nullptr;
+
+  // the commandline property is constructed as an array of int32_t
+  // followed by a series of null-terminated strings:
+  //
+  // [argc][offsetargv0][offsetargv1...]<workingdir>\0<argv[0]>\0argv[1]...\0
+  // (offset is from the beginning of the buffer)
+
+  static char desktopStartupPrefix[] = " DESKTOP_STARTUP_ID=";
+
+  int32_t argvlen = strlen(cwdbuf);
+  for (int i = 0; i < argc; ++i) {
+    int32_t len = strlen(argv[i]);
+    if (i == 0 && aDesktopStartupID) {
+      len += sizeof(desktopStartupPrefix) - 1 + strlen(aDesktopStartupID);
+    }
+    argvlen += len;
+  }
+
+  auto* buffer = (int32_t*) malloc(argvlen + argc + 1 +
+                                   sizeof(int32_t) * (argc + 1));
+  if (!buffer)
+    return nullptr;
+
+  buffer[0] = TO_LITTLE_ENDIAN32(argc);
+
+  auto *bufend = (char*) (buffer + argc + 1);
+
+  bufend = estrcpy(cwdbuf, bufend);
+
+  for (int i = 0; i < argc; ++i) {
+    buffer[i + 1] = TO_LITTLE_ENDIAN32(bufend - ((char*) buffer));
+    bufend = estrcpy(argv[i], bufend);
+    if (i == 0 && aDesktopStartupID) {
+      bufend = estrcpy(desktopStartupPrefix, bufend - 1);
+      bufend = estrcpy(aDesktopStartupID, bufend - 1);
+    }
+  }
+
+#ifdef DEBUG_command_line
+  int32_t   debug_argc   = TO_LITTLE_ENDIAN32(*buffer);
+  char *debug_workingdir = (char*) (buffer + argc + 1);
+
+  printf("Sending command line:\n"
+         "  working dir: %s\n"
+         "  argc:\t%i",
+         debug_workingdir,
+         debug_argc);
+
+  int32_t  *debug_offset = buffer + 1;
+  for (int debug_i = 0; debug_i < debug_argc; ++debug_i)
+    printf("  argv[%i]:\t%s\n", debug_i,
+           ((char*) buffer) + TO_LITTLE_ENDIAN32(debug_offset[debug_i]));
+#endif
+
+  *aCommandLineLength = bufend - reinterpret_cast<char *>(buffer);
+  return reinterpret_cast<char *>(buffer);
+}

new file mode 100644
--- /dev/null
+++ b/widget/xremoteclient/RemoteUtils.h
@@ -0,0 +1,14 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/. */
+
+#ifndef RemoteUtils_h__
+#define RemoteUtils_h__
+
+char*
+ConstructCommandLine(int32_t argc, char **argv,
+                     const char* aDesktopStartupID,
+                     int *aCommandLineLength);
+
+#endif // RemoteUtils_h__
\ No newline at end of file

--- a/widget/xremoteclient/XRemoteClient.cpp
+++ b/widget/xremoteclient/XRemoteClient.cpp
@@ -5,16 +5,17 @@
 /* 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/. */
 
 #include "mozilla/ArrayUtils.h"
 #include "mozilla/IntegerPrintfMacros.h"
 #include "mozilla/Sprintf.h"
 #include "XRemoteClient.h"
+#include "RemoteUtils.h"
 #include "plstr.h"
 #include "prsystem.h"
 #include "mozilla/Logging.h"
 #include "prenv.h"
 #include "prdtoa.h"
 #include <stdlib.h>
 #include <unistd.h>
 #include <string.h>
@@ -597,95 +598,30 @@ XRemoteClient::FreeLock(Window aWindow)
       return NS_ERROR_FAILURE;
   }
 
   if (data)
       XFree(data);
   return NS_OK;
 }
 
-/* like strcpy, but return the char after the final null */
-static char*
-estrcpy(const char* s, char* d)
-{
-  while (*s)
-    *d++ = *s++;
-
-  *d++ = '\0';
-  return d;
-}
-
 nsresult
 XRemoteClient::DoSendCommandLine(Window aWindow, int32_t argc, char **argv,
                                  const char* aDesktopStartupID,
                                  char **aResponse, bool *aDestroyed)
 {
   *aDestroyed = false;
 
-  char cwdbuf[MAX_PATH];
-  if (!getcwd(cwdbuf, MAX_PATH))
-    return NS_ERROR_UNEXPECTED;
-
-  // the commandline property is constructed as an array of int32_t
-  // followed by a series of null-terminated strings:
-  //
-  // [argc][offsetargv0][offsetargv1...]<workingdir>\0<argv[0]>\0argv[1]...\0
-  // (offset is from the beginning of the buffer)
-
-  static char desktopStartupPrefix[] = " DESKTOP_STARTUP_ID=";
-
-  int32_t argvlen = strlen(cwdbuf);
-  for (int i = 0; i < argc; ++i) {
-    int32_t len = strlen(argv[i]);
-    if (i == 0 && aDesktopStartupID) {
-      len += sizeof(desktopStartupPrefix) - 1 + strlen(aDesktopStartupID);
-    }
-    argvlen += len;
-  }
-
-  auto* buffer = (int32_t*) malloc(argvlen + argc + 1 +
-                                      sizeof(int32_t) * (argc + 1));
-  if (!buffer)
-    return NS_ERROR_OUT_OF_MEMORY;
-
-  buffer[0] = TO_LITTLE_ENDIAN32(argc);
-
-  auto *bufend = (char*) (buffer + argc + 1);
-
-  bufend = estrcpy(cwdbuf, bufend);
-
-  for (int i = 0; i < argc; ++i) {
-    buffer[i + 1] = TO_LITTLE_ENDIAN32(bufend - ((char*) buffer));
-    bufend = estrcpy(argv[i], bufend);
-    if (i == 0 && aDesktopStartupID) {
-      bufend = estrcpy(desktopStartupPrefix, bufend - 1);
-      bufend = estrcpy(aDesktopStartupID, bufend - 1);
-    }
-  }
-
-#ifdef DEBUG_bsmedberg
-  int32_t   debug_argc   = TO_LITTLE_ENDIAN32(*buffer);
-  char *debug_workingdir = (char*) (buffer + argc + 1);
-
-  printf("Sending command line:\n"
-         "  working dir: %s\n"
-         "  argc:\t%i",
-         debug_workingdir,
-         debug_argc);
-
-  int32_t  *debug_offset = buffer + 1;
-  for (int debug_i = 0; debug_i < debug_argc; ++debug_i)
-    printf("  argv[%i]:\t%s\n", debug_i,
-           ((char*) buffer) + TO_LITTLE_ENDIAN32(debug_offset[debug_i]));
-#endif
-
+  int commandLineLength;
+  char* commandLine = ConstructCommandLine(argc, argv, aDesktopStartupID,
+                                           &commandLineLength);
   XChangeProperty (mDisplay, aWindow, mMozCommandLineAtom, XA_STRING, 8,
-                   PropModeReplace, (unsigned char *) buffer,
-                   bufend - ((char*) buffer));
-  free(buffer);
+                   PropModeReplace, (unsigned char *) commandLine,
+                   commandLineLength);
+  free(commandLine);
 
   if (!WaitForResponse(aWindow, aResponse, aDestroyed, mMozCommandLineAtom))
     return NS_ERROR_FAILURE;
   
   return NS_OK;
 }
 
 bool

--- a/widget/xremoteclient/moz.build
+++ b/widget/xremoteclient/moz.build
@@ -5,10 +5,11 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 with Files("**"):
     BUG_COMPONENT = ("Core", "Widget")
 
 FINAL_LIBRARY = 'xul'
 
 SOURCES += [
+    'RemoteUtils.cpp',
     'XRemoteClient.cpp',
 ]