User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:59.0) Gecko/20100101 Firefox/59.0 Build ID: 20171124151312 Steps to reproduce: 1. Set Mint-Y as the default theme for GTK3 windows. *Mint-Y is one of the default themes in Linux Mint 18, so you may wish to test this in Linux Mint AND Cinnamon before using the theme in other Linux distros and desktop environments. 2. Restart Firefox. 3. Go to Menu > Customize, and make sure that the "Title Bar" option is unchecked. Actual results: The window control buttons (i.e. Close, Maximize and Minimize buttons) have a weird shadow unlike in other windows. Even worse, these buttons are not shown when a custom theme is applied (or changing the theme to other than 'Default'). Expected results: The buttons should be rendered properly as with other GTK3 themes, such as Adapta and Adwaita.

Probably a duplicate of [1408335](https://bugzilla.mozilla.org/show_bug.cgi?id=1408335)

Can you please check if Bug 1408335 fixes that for you? It should be in latest nightly already.

So far, the bug is fixed, even though small details such as window corners are not present. However, when applying the Firefox's Light theme, there is a white bar at the top of the tab selection. Is that intentional?

Here's what the browser look like in Elementary OS (with OSX-Arc theme and window buttons tweaked to mimic macOS). Is there a bug related with matching the window control buttons with GTK (Cinnamon, GNOME, Unity, XFCE and forks), QT (KDE and LXQT) as well as Openbox (LXDE) order?

Controls are covered by Bug 1419442. The white strip does not have a bug yet but should be also partially fixed by Bug 1419442.