ClientPortBase.Response has type StringBuilder.
Row 551 of FireFoxClientPort.cs: Response.Append(LastResponse);
So each response append char sequence to this string builder and newer clear it. In my case it tooks 2 GB of memory after 30 minuts of work.
I don't see any reason to keep this member since it doesn't use anywhere and i cannot imagine when i could use all my responses. I just replaced it by strigng and it works ok for me.