I think it takes up too much screen real estate, and doesn’t add anything to the view.
Right- click on menubar, select “customize toolbar”, select " titlebar" on left bottom. See here: https://www.reddit.com/r/kde/comments/udvxje/why_are_the_window_decorations_for_firefox/
I think there is an option if you right click the toolbar and the go to customize toolbar
https://www.geeksforgeeks.org/techtips/customize-toolbar-firefox/
Well it’s not just Firefox, it’s all windows.
Idk about Mint in particular. There’s a way to do this in most de/wm. I would start in the system settings where you can choose light/dark themes. Then just go top to bottom in the system settings.
Sorry I couldn’t help more
Edit: If you don’t go through settings systematically like this often, it’s good practice. You always find things you’ll use later.
Right click on tab bar > click on last entry there (forgot how it’s called) and finally uncheck second entry from bottom left.
