There seems to be a specific reason why the feature 'Insert signature here' is not available in Thunderbird.
This FAQ entry from the Thunderbird add-on Signature Switch (suggested by Horatio) actually provided me with some insight as to why Thunderbird probably doesn't have this feature.
FAQ Question: My signature doesn't get removed when
it is placed before quoted text. Is
that a bug?
No it isn't. ;-)
If your Thunderbird is set to place the signature below your reply
(above the quote) the removal won't
work.That's because Thunderbird then
doesn't insert the standard-delimiter
("-- \n") before the signature (for a
good reason!). And that's what
"Signature Switch" is looking for.
Some people might try to bypass that problem by manually adding the
sig-delimiter to their signature file.
But that will cause another problem.
The "-- \n" always marks the very end of a mail-message. "Signature
Switch" assumes that any text placed
below the signature-delimiter
(including any quoted lines) is part
of the signature. ... and therefore
removes everything below the
Besides... this option isn't recommended by Thunderbird itself! So
please put your signature below your
quotings. (check the Wikipedia-excerpt
here is the excerpt:
From Wikipedia: " ... The formatting of the sig block
is prescribed somewhat more firmly: it
should be displayed as plain text in a
fixed-width font (no HTML, images, or
other rich text), and must be
delimited from the body of the message
by a single line consisting of exactly
two hyphens, followed by a space,
followed by the end of line (i.e., "--
\n"). ... "
FAQ Question: I want to place my signature at any
desired position within the message.
Could you please implement
cursor-point-insertion to "Signature
Definitely no. Sorry. (And please don't ask me to reconsider.)
Due to the reasons explained for the previous question I can't offer
you this. Besides... something like
that wouldn't have anything to do with
If you're looking for an easy way to paste predefined text please check
out other extensions especially made
for that purpose.
Still, that is all fine and correct from a technical point of view, but usability-wise I just would like to insert my signature where I want.