Sorry, this is a new feature in beta and there's no preference to disable it. There were discussions about it on r/Firefox yesterday, but I don't recall whether anyone found a workaround. My only thought for preservation of sanity is to put a dummy tab in the first position to block yourself from accidentally dragging a tab you care about into the pinned tab region. Ref. * https://www.reddit.com/r/firefox/comments/1mw4m0k/ive_been_at_work_for_an_hour_but_ive_already/ * https://www.reddit.com/r/firefox/comments/1mvgkfl/tabs_collapsing_when_dragged_to_left_end/