You can modify this yourself.

1. Go to the source your Firefox profiles folder (on xp it's like : c:\documents and settings\username\application data\mozilla\firefox\profiles\random-crap.default\extensions\simplemail@telega.phpnet.us\chrome\content)
2. Using any text editor (ConTEXT is my fav), edit utils.js
3. Around lines 25-31, rearrange the date-part bits as you see fit. I merely swapped the relevant parts of lines 25-26 to get American English m/d/y format. Here's my code for those lines:

return SmUtils.pad(date.getMonth() + 1) + "/" +
SmUtils.pad(date.getDate()) + "/" +
date.getFullYear().toString().substr(2) + " " +
SmUtils.pad(date.getHours()) + ":" +
SmUtils.pad(date.getMinutes()) + ":" +
SmUtils.pad(date.getSeconds());
},

4. Depending on your setup, you'll have to either clear your cache+refresh SimpleMail and/or restart FireFox to see the changes.

Enjoy.