In Thunderbird v115, pressing Ctrl+Shift+K (in Windows) displays the Quick Filter bar (if it's not currently visible). However, the same shortcut does not hide or toggle the display of the Quick Filter bar. It can be hidden by navigating through the menus to find the option and unchecking it, but it might be more intuitive and convenient to have the same keyboard shortcut toggle the display of the filter bar instead of only displaying it. Or perhaps a small "X" icon at the right edge of the toolbar could be added to allow hiding it using the mouse.