What do you mean by "letter"? A single character, or an email message? Some users do refer to email messages as "letters" or even "notes".

In general, a computer has two input modes. The default is to insert (not overwrite) at the cursor position. (Note that in many cases, you may have two cursors; one is the pointer driven by the mouse, and the other is shaped like a capital I or a straight vertical line and represents the text insertion point. You can set the insertion point with the mouse cursor but not vice-versa.) The other mode is "overwrite" mode where the existing text is just replaced by the new typing. Generally, the "Insert" key toggles between these two modes. Some programs will show you the current state of the Insert mode in the status bar; sadly, Thunderbird appears not to. You may also have an LED on your keyboard to help out.

On top of all that, in most editing environments, if you have selected any text before you start typing, the new text will replace ALL of the selected text.

Sorry I was not clear. By "letter" I did mean a single character. I suspect you have identified and solved my problem, I was in "overwrite" mode in Thunderbird and you get no indication of this....

I'll try toggling on the insert key...