If you are talking about the web interface, I think you type your reply where indicated and then press Reply or Send. Testing it now - pressing Send.
[firebird-support] Re: replying...
hele...@iinet.net.au [firebird-support] Mon, 14 Aug 2017 12:31:16 -0700
- [firebird-support] replying... prosse...@yahoo.com [firebird-support]
- [firebird-support] Re: replyi... hele...@iinet.net.au [firebird-support]
- [firebird-support] Re: re... prosse...@yahoo.com [firebird-support]