A google response 
To view new messages at the top of a conversation thread in Thunderbird, you need to ensure your "Sort by" setting is set to "Date" with the "Descending" order; this will automatically place the newest message at the top of the conversation thread.
To view new messages at the top of a conversation thread in Thunderbird, you need to ensure your "Sort by" setting is set to "Date" with the "Descending" order; this will automatically place the newest message at the top of the conversation thread.
Upgrades = Old bugs replaced with new Bugs.