Firefox 60 introduced Client Side Decoration.
It's amazing, make Firefox fits more native, saves some vertical lines, works great... but not with KWin.

For some reason, resize and move is not possible. For move I have a workaround: Hold Alt + Left cursor, and for resize Hold Alt + Right cursor.

Whose fault is that? KWin or Firefox?

Firefox introduces CSD - OMGUbuntu
https://www.omgubuntu.co.uk/2018/05/firefox-60-download

Do resize and move work in other window managers/desktop environments (esp bare-bones window managers such as openbox)?

I know kwin has generally had troubles with CSD-enabled windows (like Chrome), though as I understand it that is usually because with CSD it is the window's (application's) responsibility to handle window management tasks like moving and resizing (and desktop placement) so that would largely be limited to Activity (and Desktop?) placement (makes me wonder if FF dropped the ball somewhere. what does it do in other environments? does it only support certain window managers/environments? trying to enable it in KDE 4 causes FF to crash, so I might try testing later)