I tried with the 2.47, i got no errors but works only the interface.
calling chrome://simplemail/content/main/main.xul
When i click on an icon nothing happen.
this is the modified .manifest file
content simplemail jar:simplemail.jar!/content/
overlay chrome://browser/content/browser.xul chrome://simplemail/content/overlay.xul
overlay chrome://navigator/content/navigator.xul chrome://simplemail/content/overlay.xul
skin simplemail classic jar:simplemail.jar!/skin/
style chrome://global/content/customizeToolbar.xul chrome://simplemail/skin/simplemail.css
locale simplemail en-US jar:simplemail.jar!/locale/en-US/