Thunderbird 115 introduced a new look. I hate it. Not only does it no longer fit in with KDE Plasma, it lost functionality due to it's new integrated search bar in the title bar.
So I went on a mission to fix it.
Go into Settings > General > Language & Appereance and uncheck the Hide system window titlebar
Toggle toolkit.legacyUserProfileCustomizations.stylesheets to true in Config Editor (bottom of Settings)
Find your profile directory via Help > Troubleshooting Information > Profile Directory (or about:profiles in same page)
This directory should have lots of files e.g. prefs.js and places.sqlite
Create directory chrome
Inside that folder, create a file named userChrome.css with the below content:
#unifiedToolbar {
display:none !important;
}
Restart Thunderbird
Enjoy!
Note if you're using the Flatpak your profile directory will be different than the one listed in the above instructions. It will be
~/.var/app/org.mozilla.Thunderbird/.thunderbird/xxxxxxxx.default/or similar.