diff --git a/testing/mozbase/mozdebug/mozdebug/mozdebug.py b/testing/mozbase/mozdebug/mozdebug/mozdebug.py
index 2430c405832..3f3a54d6be4 100644
--- a/testing/mozbase/mozdebug/mozdebug/mozdebug.py
+++ b/testing/mozbase/mozdebug/mozdebug/mozdebug.py
@@ -84,9 +84,9 @@ def get_debugger_info(debugger, debuggerArgs = None, debuggerInteractive = False
     If the debugger cannot be found in the system, returns |None|.
 
     :param debugger: The name of the debugger.
-    :param debuggerArgs: If specified, it's the list of arguments to pass to the
-     debugger. A debugger specific separator arguments is appended at the end of
-     that list.
+    :param debuggerArgs: If specified, it's the arguments to pass to the debugger,
+    as a string. Any debugger-specific separator arguments are appended after these
+    arguments.
     :param debuggerInteractive: If specified, forces the debugger to be interactive.
     '''
 
@@ -118,13 +118,14 @@ def get_debugger_info(debugger, debuggerArgs = None, debuggerInteractive = False
         ['path', 'interactive', 'args', 'requiresEscapedArgs']
     )
 
-    debugger_arguments = get_debugger_info('args', [])
+    debugger_arguments = []
 
-    # Extend the default arguments for the chosen debugger with the ones passed in, if any.
     if debuggerArgs:
         # Append the provided debugger arguments at the end of the arguments list.
         debugger_arguments += debuggerArgs.split()
 
+    debugger_arguments += get_debugger_info('args', [])
+
     # Override the default debugger interactive mode if needed.
     debugger_interactive = get_debugger_info('interactive', False)
     if debuggerInteractive:
diff --git a/testing/xpcshell/runxpcshelltests.py b/testing/xpcshell/runxpcshelltests.py
index d05ce9f3ea5..c9f73073b99 100755
--- a/testing/xpcshell/runxpcshelltests.py
+++ b/testing/xpcshell/runxpcshelltests.py
@@ -1074,11 +1074,6 @@ class XPCShellTests(object):
         self.debuggerInfo = None
 
         if debugger:
-            # We need a list of arguments, not a string, to feed into
-            # the debugger
-            if debuggerArgs:
-                debuggerArgs = debuggerArgs.split();
-
             self.debuggerInfo = mozdebug.get_debugger_info(debugger, debuggerArgs, debuggerInteractive)
 
         self.xpcshell = xpcshell