--- a/netwerk/protocol/http/Http2Compression.cpp
+++ b/netwerk/protocol/http/Http2Compression.cpp
@@ -204,18 +204,18 @@ nvFIFO::nvFIFO()
 nvFIFO::~nvFIFO()
 {
   Clear();
 }
 
 void
 nvFIFO::AddElement(const nsCString &name, const nsCString &value)
 {
-  mByteCount += name.Length() + value.Length() + 32;
   nvPair *pair = new nvPair(name, value);
+  mByteCount += pair->Size();
   mTable.PushFront(pair);
 }
 
 void
 nvFIFO::AddElement(const nsCString &name)
 {
   AddElement(name, EmptyCString());
 }