You can revert everything they have changed. Depends on what you want to change, however.

You can redesign toolbars to however you want them to be from View > Toolbars. You can revert to the message list (the main area with messages) by displaying a message list header with View > Layouts > Message list header and opening the Message list display options at the top right of the message list header and selecting the list view.

Similarly, there are toggles for everything else. Just keep clicking until you get the desired look. Try searching on the internet when you do not know where to find some option.

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.

Its fancy and all, but to this day a downgrade

• search filters where there before
• filters (for deleting or sorting) have only one level now? No if (a + b) or (c + d) anymore
• Thunderbird Conversations is broken, which is horrible
• stupid useless sidebar with no purpose but to open tabs? (Butter Robot meme here)

The rest is nice though.

I hate the top bar, it just takes up space

On the other side, is it possible to upgrade to the newer UI?

Betterbird maybe? I don't really use Thunderbird much so not 100% sure what it had before. 😁