Dragging tabs opens mission control unintentionally
When I try to drag tabs to reorder them, while I'm dragging it, it shows a small transparent preview at my mouse cursor. After about 1 second of dragging it, my Mac enters mission control as if i pressed the F3 key. When i let go, it opens the tab i dragged into a separate window. This happens even when i haven't dragged the tab out of the toolbar.
Opaite Mbohovái (2)
That sounds less like a Firefox problem and more like macOS triggering Mission Control while you drag.
I’d check System Settings → Desktop & Dock → Mission Control and look at your keyboard shortcuts and Hot Corners. Make sure Mission Control isn’t assigned to a hot corner or an accidental mouse/trackpad gesture.
Also try dragging tabs with a different mouse or with the trackpad. If it only happens with one device, you’ve probably got a gesture or button mapping triggering Mission Control.
The fact that Firefox then opens the tab in a new window is just what happens when the tab drag gets interrupted.
I was trying to reproduce for a bit and then found out that when resting more fingers on the trackpad and moving them slightly while dragging it's activating the 3–finger exposé that you might want to look into disabling in your system settings for comparison:
Nde eikéke nde mba’etepe embohovái hag̃ua ñe’ẽmondo. Ikatúpa, emoñepyrũ peteĩ porandu, ndereguerekói gueteriramo nemba’ete.