When the user replied to emails ST4 would reply with the "common" template
instead of with the correct template (determined by the email address in the
"From" drop down)

Apparently the bug has happened on release version before as well. It is caused
by the "from" address picker which doesn't seem to have a consistent method of
storing the account key (sometimes it is stored in value or the "value"
attribute and sometimes in the "identitykey" attribute). To test whether one is
affected by this bug, use the

extensions.smartTemplate4.debug.functions.insertTemplate

debug setting and look out in error console for the line where the identityName
and key are shown.

Created an attachment (id=8041) [details]
0.9.7 prerelease 105

Should fix this issue both on Thunderbird release (36.0.1) and beta (38.0 beta
3)

This will go into the next release soon, so marking as Fixed.