I'm running Thunderbird 115 since this morning and have still problems getting along with the recent UI changes. The main toolbar now is integrated into the window titlebar. This makes it look very weird on KDE for multiple reasons:
1. The menu bar is now below the main toolbar. No other application ever has had this, this feel off and is very confusing.
2. Unless Thunderbird is maximised, there is almost no space to grab the window and drag it along (unless one decides to remove the search bar)
3. This feels very much like a GNOME application running on KDE 😞
4. Honestly, it looks ugly because this is the only window that modifies the window bar and if makes it stand out in a not good way
This might be something that some folk find pleasant, I for my part don't like it at all. Messing around with the desktop environment's window decoration is confusing and looks weird. It might work well in one, but will mess the experience up in another one. One cannot make good assumptions on how this will look on all desktop environments, so it's best to not mess around with the window titlebar at all.
I wish there would be a way to have the classical toolbar below the menu back!
I am so happy I found this solution. In Cinnamon (maybe other desktop environments too) I am used to pull windows from being maximized to get them to another screen. They will automatically un-maximize and I can drag them, all with one click. I can then on the new screen double click the title bar to maximise them. Super easy and intuitive way to throw windows around. The weird icons, search bar or anything on the title bar messes that up completely. I could chime into the general new GUI bashing but this was definitely the biggest pain.