Download and install the fonts Andale Mono Quikscript and Segoi UI Quikscript.  The Quikscript characters are all from the King Plus Quikscript font.

Open the Quikscript add-on file (current version is 0.1.5) with Firefox. Simply unzip it to see its source code. On Thunderbird, go to Tools -> Add-ons -> Install, and choose the same add-on file.

After installed, press Control + Shift + Q, use the menu item under Tools or click on the status bar panel to convert. Repeat to undo the changes.

Quikscript Add-on Screenshot

When you move the mouse over a Quikscript word, a red border is drawn around it and the original word is shown at the status bar.  Useful to find out whether the original word was capitalized, specially when reading software documentation.

The file with the list of words should be located at C:\words.txt. Example file.

One entry per line, each entry is:
<roman_word>=<quikscript_word>

<quikscript_word> is a set of character codes separated by commas.  0 being the first Quikscript character in the King Plus font.

Lots of things should be made configurable. Patches are welcome.

E-mail.