You'll be having to interrogate ThunderBird and to watch it's behaviour, then put that information back into FileMaker. Sounds like a huge mess, lots of work.
If you want to have control of the email flow, consider using any one of the email plugins for FMP. Doing this will mean that everything occurs inside FMP. Whatever changes the user makes is a change in an FMP field, so you have your data.
Definitely not the easiest of tasks to accomplish, but I figured it wouldn't be.
Thanks for clarifying what I need to do. Now it makes a bit more sense how it would communicate with Filemaker.