Hi, that sounds like it is doing what it is suppose to be doing. As a New Window is just that another instance of Firefox running could you explain more about what you are expecting please...

Can you check here to see if something you need, type full screen into search bar in the URL https://addons.mozilla.org/en-US/firefox/extensions/

Please let us know if this solved your issue or if need further assistance.

I'm expecting the folowing behaviour: in macOS when the FF-Window is in fullscreen mode, it has its own workspace (see screenshot, example with 2 windows in fullscreen). When i now press "Command-N" or open a new window via menu, this menu just lays over the original window, what is completely useless in macOS-fullscreen. The expecting behaviour would be to open the new window in a new Fullscreen-Workspace. (Hope this is explaining it better). By now i'm fixing this issue by first hitting "Command-N" and then "Ctrl-Command-F".

Could you try uploading that Screenshot again please.

sorry, here they are!