Have you tried installing the cups-pdf package:
sudo apt-get install cups-pdf
You'll then find if you select "Print" from any application you'll be offered "PDF" as a printer .. choose that and it will create a PDF of the document .. usually in a folder called PDF in your home folder.
If this is a Thunderbird issue, which version of Thunderbird ?
And is it Mint 13 32bit or 64bit ?