Here's my workaround since I just started using Thunderbird a few days ago. If I have an email I want to send to omnifocus, I just click cmd-E (edit message as new) and put "omnifocus" in the "TO" box. I've set a contact called "OMNIFOCUS" that is my email
xxxx+omnifocus@domain.com. SInce I use mail.app for my work email (and t-bird for my personal) when it hits mail.app, it's added to omnifocus.