Can't switch Firefox windows using command+tilde I prefer using multiple Firefox windows. Until now I could easily switch between its different windows by using the command + ~ shortcut. But all of a sudden I can't do that anymore.

I can still use the shortcut for other apps but not for Firefox. Both the windows are in the same space and I don't recall making any changes. 

Any hints on what's happened or how I can rectify this?
FWIW, I fixed the problem the following way.

I noticed that the ⌘+~ stops at a specific window, while works on other Firefox windows.

I closed that window, then re-opened it via History menu. The effect was gone, and ⌘+~ started working as expected.