2 Firefox windows - unable to close one of them

[podkilla]

I opened a tab group in another window by accident clicking Right Click -> Open group in new window

Now i would like to close this window again to have firefox in a single window as is was before. I closed all tabs within the second window but the last tab wont close the window as usual.

Closing Firefox closes both windows, starting Firefox opens both windows, an empty one and the one with all my tabs in it.

[sjdrc]

I'm encountering this exact same issue and have not been able to find any other reference to it or relevant settings to fix this behaviour. I have 5 extra open windows that cannot be closed.