This does work in IE and chrome but the pictures don't show in Firefox. What went wrong?

mypic1 = Import["ExampleData/lena.tif"];
mypic2 = Plot3D[Sin[x y ], {x, -3, 3}, {y, -9, 9}];

Export a grid with 2 pictures. Move the pictures to localdisk dumpic and put a future cloud reference folder/subfolder in the html file.

str4 =
  ExportString[
   Grid[{{"lefttop", "righttop"}, {mypic1, mypic2}}],
   "HTMLFragment",
   "FullDocument" -> True,
   "URIHandler" -> "Export",
   "FilesDirectory" -> "e:\\folder\\dumppic",
   "FilesPrefix" -> "folder/subfolder"];

Read the pictures and move them to the cloudfolder folder/subfolder as referenced by the html page.

CopyFile[#, 
   CloudObject["/folder/subfolder/" <> Last[FileNameSplit[#]]]] & /@ 
 FileNames["*", "e:\\folder\\dumppic"]

Okay now copy the html file to the root in the cloud

CloudExport[str4, "HTML", "/test.html"]

The source html code contains: src="folder\subfolder\24f48a2b629a3253.png" with slashes backwards. It seems like chrome and IE have no problem translating forward to backward slashes on the fly but firefox does not find the pictures.

How can I make sure the picture references are in the html file as forward slashes?