User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:67.0) Gecko/20100101 Firefox/67.0
Steps to reproduce:
Tested on both linux mint 19.1 with cinnamon, and on kubuntu 19.04.
Firefox version 67.0.1
1: Restart with extensions disabled.
2: Open the Toolbar customization page, uncheck "title bar"
3: Maximize firefox
The close button isn't placed all the way in the top right corner. It stops a few pixels short of the edges, both top and side.
The close button should be all the way in the corner, like it is on windows, and on linux with the title bar enabled. To quote Raymond Chen: "The corners of the screen are very valuable, because users can target them with very little effort. You just slam the mouse in the direction you want, and the cursor goes into the corner." in case someone has forgotten why it should be in the corner.