Keyboard input only works in the most recently created window
Every time I open a new window in Firefox, all existing windows lose keyboard focus. Mouse buttons work in the windows, but no keyboard input. This applies to everything in the window, including the search entry, URL entry, any entry widgets on the page, or terminals in VNC sessions on the page. It applies to all tabs in the windows.
This is a very annoying problem as I like to keep multiple Firefox windows open on multiple screens for different applications.
I am using Firefox 55.0.3 (64 bit) on a Linux Fedora Core 21 system.