--- a/netwerk/protocol/http/nsHttpChannel.cpp
+++ b/netwerk/protocol/http/nsHttpChannel.cpp
@@ -3733,17 +3733,17 @@ nsHttpChannel::OpenCacheEntryInternal(bo
             getter_AddRefs(cacheStorage));
     }
     else {
         bool lookupAppCache = (mChooseApplicationCache || (mLoadFlags & LOAD_CHECK_OFFLINE_CACHE)) &&
                               !mPostID &&
                               MOZ_LIKELY(allowApplicationCache);
         // Try to race only if we use disk cache storage and we don't lookup
         // app cache first
-        maybeRCWN = !lookupAppCache;
+        maybeRCWN = (!lookupAppCache) && mRequestHead.IsSafeMethod();
         rv = cacheStorageService->DiskCacheStorage(
             info, lookupAppCache, getter_AddRefs(cacheStorage));
     }
     NS_ENSURE_SUCCESS(rv, rv);
 
     if ((mClassOfService & nsIClassOfService::Leader) ||
         (mLoadFlags & LOAD_INITIAL_DOCUMENT_URI))
         cacheEntryOpenFlags |= nsICacheStorage::OPEN_PRIORITY;