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That's it!!! I tried to use the html code you suggested <br> and after ticking the html format box at the top of the signature box, it works just fine now. Thank you for your add on and for your time and good advice. Unlike soem of the reviewers, I think it's a great feature to be able to decide when you want your signature to be inserted or not. Cheers,
R.

这个评论是针对附加组件的一个早期版本的(0.2)。  对这个附加组件,此用户曾发表过4个评价

Perfect! Thank you for using add-signature :-)
I think this feature should be a default of thunderbird.