When replying to a message, some people prefer to add their comments above the original message, while others prefer to add their comments below the original message. Thunderbird can be configured to position the editing cursor either above or below the original message.
The position of the cursor depends on the Composition & Addressing settings for the account. To access these settings, select ToolsEdit > Account Settings
When you reply to a message, the editing cursor will automatically be placed below the original message.
If you change the Composition & Addressing setting to start my reply above the quote, the cursor will be inserted above the original message.