Firefox second window unresponsive

So I've got 6 tabs open in Firefox, all working properly. To help me focus on another task, I open a 2nd window in Firefox. It is, however, blank and unresponsive, doesn't show any extensions in the toolbar, but does show "Firefox View" which I don't normally have showing. I can only close the new window by using Task Manager to shut Firefox down completely or by right-clicking in the Taskbar on the dead window and selecting "close window". What's going on here? First image: working tabs, Second image: unresponsive new window. Regards, Ian Firefox 140.0b2 (64-bit) Windows 10 Pro