Well I don't know what I can respond... Indeed, freecad doesn't do gcode yet. That's being worked on, though. As for loosing your document tab when opening a browser tab, if it is indeed what is happening to you, it might be a bug.

But I think maybe  freecad is still a bit rough for your needs, it is a young program, and it will still take time before its interface reaches a state where it is totally friendly and usabe to everyone.