When I use Firefox in fullscreen-mode (by pressing f11) and I hover my mouse on the top of the screen to display the navigation/bookmarks bar, the app shows minimise/maximise/quit buttons that clearly belong to Windows (see image below). Even more interestingly, they work like you expect them to work. This isn't bothering me in any way, but I'd like to know why this is possible and how Windows-style buttons have ended up in Linux/eOS.
Explanation comes from your theme. I suppose you are not using the default Elementary OS themes, but the different one.
There is also explanation about the firefox title bar below.
How to get rid of huge firefox title bar.
Just click the top-right menu (the hamburger(☰) menu) > Customize, you’ll be at the Customize Firefox page, on the bottom-left most, unchecked the Title Bar. That’s it 🙂
Additionally if you will install Elementary OS Tweaks and open it, go to Appearance, you will have a layout which can be changed. You can choose from the dropdown menu which layout you want to use.
Eventually Firefox has these buttons built-in. Will check this later and update thread.