You wont be able to revert everything, since 115 is essentially a new underlying UI framework. Your best bet is a fork like betterbird since you’ll be able to just load up your profile and keep going fairly seamlessly. But chances are rheyll eventually catch up to 115, and betterbird may diverge further or follow along.

Thunderbird 115 does have a decent amount of customization still. You can switch the list views from the new cards look back to the table layout, if that is your main dislike. It’s the icon at the top right of the inbox list next to the quick filters button. You can also change the layout to a horizontal one from the hamburger menu under views.

As far as tool bars there is not much you can do other than try to get as close to your preferred setup with the customize capabilities. Just right click on the top header area, just like in firefox. It wont have everything from older versions of Thunderbird, but most of the common tasks are available to use there instead of in the message veiwer or on the list/table view.

And of course, there may be some add-ons which restore some UI elements or features you miss. Many of those things would need to be re-implemented for the newer UI framework and would need enough people who use them to justify an add-on dev or the thunderbird team to make it happen.