Having this same issue and I haven't been able to find a solution yet
Upgraded from High Sierra.
[doublepost=1537842595][/doublepost]So I think I found the issue, based on the fact that I didn't have it right after the upgrade. I didn't like the full-dark mode of Mojave at first so I tried the command from this post.
I have noticed that in Mojave the Firefox browser window now appears in all of my desktop spaces. In previous Mac OS versions it only appeared in the desktop space I opened it in.
Has anyone else noticed this? BTW if this is seen as off topic let me know and I will post it elsewhere.