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.

To restore the regular title bar:

Go into Settings > General > Language & Appereance and uncheck the Hide system window titlebar

To remove search bar:

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!