Using Mac OSX El Capitan 10.11.6 and Firefox 75.15.0 esr I have a number of desktops defined in "spaces" and each app goes into one. Something odd has happened with Firefox. When I select a different Window from the Window dropdown, or from the Dock, sometimes there is a "sliding sideways" effect. And if I go into Firefox by going to the Dock and tapping its icon - it goes to one window; BUT if go to the Dock and tap again, it changes to another window. I think, always the same two. So its sort of behaving like "spaces" does, but when I view all the "desktops" in the selection view at the top of the screen, I only see one containing Firefox windows. Its really annoying when the window changes like that, and I don't know how to get them into a single grouping. I have quit & reopened many times, but it persists. If I knew which of the Firefox Windows were in each of these glitchy "pseudo-groups" then I might quit all of one group, and then re-open them. Not sure if there are just two such pseudo-groups, or more.