There is a bug open to return its ability to remember the last-used addon.

I don't use the sidebar. I type and use auto complete. Can you describe a situation and the corresponding workflow where looking at the sidebar is useful or efficient? I'm curious. I occasionally enable the Sidebar and inevitably find that it is just cluttering up my screen. Very rarely, I might use it to address a message to a colleague where I'm not entirely sure of his email address. It's usually his initials, but may or may not include a middle initial, dependent on whether or not it disambiguates him from another colleague with similar initials.

My Contacts sidebar is showing all the time. I prefer it that way. But by default, it starts with "All Address Books" showing and it is a nuisance to have to click it to bring up the address book I want every time I go to the Write screen. There is a new right side sidebar that supports features like "Lightning" that I do not use. It just takes up space on the screen. I closed it with the "X" and the next time TB started, I noticed a bar popped up at the bottom of the screen allowing me to stop it from coming up.

What I want to do is eliminate the "All Address Books" feature and return TB Contacts sidebar to the way it has always been. Any ideas? A couple of my friends have the same complaint.

No, as I said, we have to wait for the bug to be actioned to revert to the previous state of affairs where the address book remembers which one you used last time and so opens to that address book by default. Sorry.