Update to the bug I filed that was introduced in Firefox 116 - apparently, it isn't a bug:
I don't think this is a bug, its a side effect of the behavior for the ctrl|cmd + shift + T shortcut changing in 116 to reopen the last closed tab or window, in the order closed.
Technically, you closed a window when you dragged the tab out to a window, and then back to the original window. The exact same behavior happens if you use repeat those steps and use the ctrl|cmd + shift + N shortcut to reopen the last closed window.
There's an open needinfo for the author of the regressor, so I don't have much of a comment, but... it is very clearly a bug and very clearly a regression - Chrome doesn't do this (and actually has the newly claimed Firefox behavior) -- and neither did Firefox before it regressed.