Bookmarks sidebar.
Why does Bookmark sidebar open in new window? What preferences, customize will make this stop?
ప్రత్యుత్తరాలన్నీ (1)
If you have opened the sidebar in one window then it opens in all other windows you open. You can try to close all extra windows and close the sidebar in the main window and then close (Quit) the Firefox application. If you restart Firefox then the sidebar should be gone.