This add-on can do as you ask: https://addons.mozilla.org/en-us/thunderbird/addon/mms-auto-correct/ It also does several other things that drive me to distraction. I find pressing the shift key much less hassle. There are so many places where auto capitalization doesn't happen that having to cope with its absence in email seems a very small worry. You should probably look for other email clients if this is very important to you.