Start Your Reply on Top in Mozilla Thunderbird
To make Mozilla Thunderbird position the cursor on top, above quoted text, when you reply:
Select the Menu icon in the upper-right corner and choose Account Settings. Select Composition & Addressing in the left sidebar. Make sure Automatically quote the original message when replying is checked, then select start my reply above the quote next to When quoting.
Alternate Mozilla Thunderbird Reply Options
Mozilla Thunderbird automatically places the cursor above quoted text when you start a reply. Instead of moving the cursor every time, you can change where the cursor appears by default:
Start my reply below the quote: Mozilla Thunderbird quotes and indents the text of the message to which you reply. The first line of the reply introduces the quoted text, and the text cursor is placed below the quoted message for you to start typing, above your signature (if you set up a Thunderbird signature). Select the quote: Mozilla Thunderbird quotes the original message in your reply. The first line introduces the original text as a quotation, and your signature is inserted below the quoted text, after some space for your reply. All text (not just the quoted text) is highlighted.
