Rated 5 out of 5 stars

Here is a sentence based on candidcactus' example (better than mine):
i WAS VISITING THE sOVIET uNION LAST jULY AND absolutely NO aMERICANS WERE IN sT pETERSBURG THAT DAY.

and here it is after employing Joshua's newly-added tOGGLEcASE option, Alt-9:
I was visiting the Soviet Union last July and ABSOLUTELY no Americans were in St Petersburg that day.

In other words, with the touch of two keys, the cases have been toggled, just as we wished for. Thank you, Joshua--well done! I hope the same feature will soon be added to TitleCase for Firefox.

Lenny

This review is for a previous version of the add-on (2.0). 

Rated 4 out of 5 stars

This is a great little add-on and does just what it promises, but I too, especially on my new HP kbd with the slightly misplaced Caps Lock key, have a tendency to hit Caps Lock mid-sentence, someTIMES MIDWORD, AND THEN FINISH THE SENtence before I look up and see what I've done. Yes, it can usually be repaired with the lowercase option, but if the phrase contains capitalized and/or all-caps words you'd have to select the entire phrase and choose lower case, then select the capitalized words and choose Start Case, and finally choose the all-caps words and choose UPPER CASE . Surely it can't be that much trouble to add a one-step tOGGLE cASE function to TitleCase, especially as most of your reviewers so far seem to be fat-fingered klutzes who all make the same typing error. (no offense, guys)

This review is for a previous version of the add-on (2.0). 

You can try out the 2.5 version here with tOGGLE cASE: https://addons.mozilla.org/en-US/thunderbird/addon/titlecase-for-thunderbird/versions/