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:
- Quit Application -- choose Thunderbird
- Get Selected Finder Items
- Run Shell Script -- Choose Shell: /bin/bash and Pass input as arguments, then enter the following script:
for f in "$@" do /Applications/Thunderbird.app/Contents/MacOS/thunderbird-bin -compose attachment="file:///$f" > /dev/null 2>&1 & done
- Save the workflow with a name like 'Mailme' and you are done.
- You can now select a Finder item and have it attached to a new mail message in Thunderbird.
[crarko adds: I haven't tested this one. If you spot any errors or improvements please note them in the comments.]

