Recently I started having this issue. Whenever I send a tab from my phone into my computer, instead of a new tab opening in whatever window I was last using, a new private window opens and then loads the tab. At first I was only transferring one or two tabs so I didn't really care at the time, copy and paste the tabs into the old window. But when I need to transfer 10+ tabs? how aobut 40? Now it's an issue since you can't just move them. The tabs in my phone are normal too, not private. I have, of course, found no normal setting in either the desktop or android version of Firefox to control this behavior.