Is it customizable? That seems like a pretty strange question from someone that maintains it for a user base. Apart from "disabling" tabs what are you looking to customize? Having used TB since inception I might think a more apt question is "What can't you customize?".

I also don't think I understand your tab stance. You don't have to base your usage on tabs, do you? Personally, given the volume and types of mail I deal with, I was waiting for tabs for years. I just want the context menu option to "view message in conversation" to always exist. It dropped off a couple of builds ago and I can't seem to get it back. Usability of mail without this is decreased exponentially.