--- a/toolkit/crashreporter/tools/symbolstore.py
+++ b/toolkit/crashreporter/tools/symbolstore.py
@@ -310,17 +310,18 @@ def validate_install_manifests(install_m
 def make_file_mapping(install_manifests):
     file_mapping = {}
     for manifest, destination in install_manifests:
         destination = os.path.abspath(destination)
         reg = FileRegistry()
         manifest.populate_registry(reg)
         for dst, src in reg:
             if hasattr(src, 'path'):
-                file_mapping[os.path.join(destination, dst)] = src.path
+                abs_dest = os.path.normpath(os.path.join(destination, dst))
+                file_mapping[abs_dest] = src.path
     return file_mapping
 
 def GetPlatformSpecificDumper(**kwargs):
     """This function simply returns a instance of a subclass of Dumper
     that is appropriate for the current platform."""
     # Python 2.5 has a bug where platform.system() returns 'Microsoft'.
     # Remove this when we no longer support Python 2.5.
     return {'Windows': Dumper_Win32,