--- a/netwerk/protocol/http/Http3Stream.cpp
+++ b/netwerk/protocol/http/Http3Stream.cpp
@@ -71,17 +71,18 @@ bool Http3Stream::GetHeadersString(const
   FindRequestContentLength();
   return true;
 }
 
 void Http3Stream::FindRequestContentLength() {
   MOZ_ASSERT(OnSocketThread(), "not on socket thread");
   // Look for Content-Length header to find out if we have request body and
   // how long it is.
-  int32_t contentLengthStart = mFlatHttpRequestHeaders.Find("Content-Length:");
+  int32_t contentLengthStart =
+      mFlatHttpRequestHeaders.Find("content-length:", /* aIgnoreCase = */ true);
   if (contentLengthStart == -1) {
     // There is no content-Length.
     return;
   }
 
   // We have Content-Length header, find the end of it.
   int32_t crlfIndex =
       mFlatHttpRequestHeaders.Find("\r\n", false, contentLengthStart);