This is all down to what you are doing, so clearly your technique is not being precise enough and/or your computer keyboard/mouse settings are not correctly set up to suit you. The following information is standard in all text processing applications. You cannot just stop a main part of functionality. This information should help to explain what you are doing to get the various results. '''To insert a letter:''' eg: mistke mist'''a'''ke You would need to position cursor in the tiny space between the 't' and the 'k' and use single left click to set the flashing cursor position and then type 'a'. '''If you position cursor in the space between letters and slightly move the cursor over an adjacent letter whilst doing a single left click,''' then it will highlight the actual letter because that is what you instructed. This is ok if you wanted to change the letter from a 'm' to say 'n'. Or wanted to delete that highlighted letter by then using 'backspace' key. You would need to be careful that you are not moving the cursor as you left click. '''If you double left click on a word,''': then the whole word will become highlighted, which is fine if wanted to remove that word and retype or wanted to select that word to change the format eg: make that word bold. '''If you double left click at the end of line''' : this is deemed as a blank area or anywhere in the blank text area then you are instructing it to highlight everything. A single left click at the end of a line will do nothing other than position the cursor for typing. ------------------------------ You have not mentioned whether you are using a mouse or a touchpad. Touchpads can be much more sensitive and not so easy to control. It sounds like you are too easilly double clicking, so perhaps your settings are not tuned to suit you. On your computer - you did not say what Operating System you are using eg: Windows Vista, Windows 7 etc Start > Settings > Control Panel You should see 'Mouse' and possibly 'Touchpad' Double click to open 'Mouse' there should be a means of setting the speed of a 'double click' Usually, it says 'speed' slow ---- fast and offers a slidebar to make slower/faster There is usually an image of a folder which you use to test the double click. It would visually look closed, double click on it to see it look like it is open. Alter the speed to suit you. You might need to make it a bit slower. Because at the moment your single click is probably pressing the sensor twice in rapid succession, your left click is too long, so the keyboard and computer think you have double clicked. This then sends that instruction to Thunderbird. Hope this info helps you get to grips with controlling the mouse. Another technique may suit you better and that is to use the keyboard arrow keys to move about. Left, Right Up and Down. So if had written: eg: mistke and the cursor was at the end of that mispelt word, I could use the left arrow key twice to move the cursor back and positioned in between the 't' and 'k' and then press the 'a' key. If you wanted to then move the cursor to the end - no need to keep pressing the right arrow key, press the 'End' key to jump cursor to end of that line.