External links suddenly open in a new window instead of a tab

I am using Nightly (20.0a1) on WIndows 8, I use the program Teamspeak 3 and it used to be that whenever I clicked on a link it would just open in a new tab in the Nightly window that was open, but now it opens a completely new Nightly window. I have the box ticked in settings, I've gone through all of the about:config things I've seen online and nothing works, I really have no idea what to do. I just did something with envoirenmental variables which didn't help (meant that it opened a whole new instance of Nightly whenever I clicked on an external link) so I deleted the variable. Anyone got any suggestions? If it is relevant, before this problem started I dragged a tab from one window of Nightly to another (think it might've been the cause of this), and yes, I've tried dragging tabs around from window to window to fix this and it didn't work.

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