For some reason, Mozilla Thunderbird defaults so that when you hit the reply button, your cursor starts at the bottom of the message, underneath the quoted text. Now this kind of reply was all well and good in the usenet days, but in modern email, your reply should be at the top of the message.
Here is how to change the default behavior in thunderbird:
Go to Tools > Options:
Click on the Config Editor button and type reply into the Filter box:
Double Click on the one that says “mail.identity.default.reply_on_top” and change the value from 0 to 1:
Click on Ok and restart Thunderbird.
Now it will automatically place the cursor at the beginning of all replys for all acounts by default.