In Thunderbird's templates folder I have several templates. Since version 2 of
smarttemplates when I create new message from template signature is removed. 

Everything works fine when smarttemplate is disabled.

This may bee a side effect from 
[Bug 26536] Support using SmartTemplate4 variables in Thunderbird Templates (Tb
60)

I have been working on this problem today. I want thee %sig% variable also to
work when used within a thunderbird temeplate (just the same way as it is in
Stationery)

Created an attachment (id=8575) [details]
2.1 pre 17

This version allows using the variables from [Bug 24993] and also %sig()% 
within a Thunderbird template.

Supported variables:

%subject%
%from%
%to% %cc% %bcc%
%identity%
%date%
%dateformat()%
%sig%

If %sig% is omitted then it should be treated according to the Thunderbird
Composer account settings. To force omission of signature, add %sig(none)%. To
determine a specific position *within* the template, use %sig(html)%.

The variables from above (except %sig% itself) should also usable within the
signature HTML.

Resolved Fixed: Release 2.1 - 28/07/2019