I would like to use the same template for New and Forward/Reply emails with: 
Dear %from(firstname)%, [[CC: %cc(name)%,]]

For New emails I get a smarttemplate4 warning message window (translated to
English by me) "The variable %from% can not be used for new messages. For a
list of usable variable see help"
There it says for : "Some variables cannot be used when composing new mails; a
message will be displayed should you chose one accidentally."

I'm not sure if to mark this as bug or feature request but I would love if one
or both of the requests are interesting to solve: 
a) the warning message window gets no own window in my Win7 but instead seems a
Thunderbird instance window meaning I can not easily reach it to click OK by
clicking two times on the TB taskbar symbol when I have opened many programs in
the taskbar. I hope this is understandable. 
b) Why not allow/ignore %from% and similar variables for New emails? When I
click OK in the warning message window I can continue without problems. If this
warnings are useful for template error debugging maybe make them an option to
activate?

If this is about supporting %from% in new emails - it is definitely a feature
request. 

And not an easy one - because you are asking for something that doesn't exist.
The from header of an email relates to the sender of the email we are replying
to (like most of the header fields that SMartTemplate4 offers.)

As regards the error message, it is used when clicking on the variables in the
SmartTempalte4 settings Advanced > Variables section (where it behaves better).
I need to look at how I can improve it when it is called during composer.

Created an attachment (id=8154) [details]
1.2 prerelease 36

Try this version - it partly fixes the weird windows stacking problem by using
the compose window as the parent for the error message. 

What essentially happens here is that an exception is thrown - it tries to
retrieve the header from the email that doesn't exist. At some stage I am
hoping to be able to put in a placeholder that can be updated when the users
adds recipients or  edits the subject line and then inserts the "future header"
of the email - it is not necessarily a given that the header exists before the
"send" button is pressed so this is an additional complication.

warning position window fixed in 1.2

if you want to support headers that aren't set yet in the "new mail" template,
please raise a separate bug