In the bookmarks toolbar, if you have too many bookmarks, it overflows with a dropdown menu at the right. If you open this up, and click one and drag, you can reorder bookmarks in it. However, when you do this, it then displays *all* the bookmarks from the toolbar in the menu. Note that sometimes it flashes and then closes itself, while sometimes it stays open.
Related to / variant of Bug 331297 or is it more recent?
Steps to reproduce: 1. Get enough bookmarks in the Bookmarks Toolbar so you see the chevron. 2. Open the dropdown menu and and click and drag a bookmark to another place in the dropdown. Results: After you release the drag the entire contents of the Bookmarks Toolbar will show up in the dropdown. Probably the same core issue as bug 331297.
I see this on Mac, too, though the menu only seems to be expanded for an eyeblink. Also annoying is the fact that when I drop the bookmark during moving, it starts to load it in the active tab, so two things are grabbing the drop. But that's probably a separate bug.
This got fixed at some point.