If it is any consolation, I am running Win7 Pro 64 with Acrobat 9.0 Std., and Thunderbird 3.0.
I have everything set to Thunderbird as my default email client.
Still, when trying to attache a PDF to an email (by clickining on the icon on the Acrobat toolbar), Acrobat can not find Thunderbird. No go.
I've posted this issue as a new thread a few days ago.
I don't think Thunderbird is a supported email client. At least it hasn't been in the past.