As I prefer Thunderbird over Apple Mail, due to it's better handling of things like status-notifications and the like, it has always bothered me that I could not select a file in the Finder and have it being used as an attachment for Thunderbird.
However, I have found a solution. It is still basic, and it may need some fine-tuning.
In Automator, create a new workflow with the following actions:
for f in "$@" do /Applications/Thunderbird.app/Contents/MacOS/thunderbird-bin -compose attachment="file:///$f" > /dev/null 2>&1 & done
Mac OS X Hints
http://hints.macworld.com/article.php?story=20101026075900608