--- a/gfx/thebes/gfxFT2FontList.cpp
+++ b/gfx/thebes/gfxFT2FontList.cpp
@@ -956,17 +956,17 @@ gfxFT2FontList::AppendFacesFromFontFile(
     uint32_t filesize = 0, timestamp = 0;
     if (aCache) {
         aCache->GetInfoForFile(aFileName, cachedFaceList, &timestamp, &filesize);
     }
 
     struct stat s;
     int statRetval = stat(aFileName.get(), &s);
     if (!cachedFaceList.IsEmpty() && 0 == statRetval &&
-        s.st_mtime == timestamp && s.st_size == filesize)
+        uint32_t(s.st_mtime) == timestamp && s.st_size == filesize)
     {
         LOG(("using cached font info for %s", aFileName.get()));
         AppendFacesFromCachedFaceList(aFileName, cachedFaceList, aStdFile);
         return;
     }
 
     FT_Face dummy = Factory::NewFTFace(nullptr, aFileName.get(), -1);
     if (dummy) {