After I have Twitch or Youtube in fullscreen mode and then double click to minimize the player the Firefox window has changed its position

As stated before. After I've exited out of fullscreen mode on YouTube and Twitch's HTLM player, the postition of the Firefox window changes. I run two Firefox windows on one screen. Both halfscreen, one left and one right. What I usually do is that I have Youtube subcription list on the left and drag the links to my right window and watch it there(and usually on fullscreen mode). After I've watched the clip and exit the right window's position goes down and to the right. I'll add a screenshot to show the position it takes. As you can see on the screenshot. The right window should stand like the left one, just on the right. Thanks in advance!