I have one for general stuff, one for when I'm working, mainly to minimise distractions, and one to do 3D work.

Each activity has 9 virtual desktops because that's what I use in the main profile.

And yeah Firefox is a bit different, however you can select an option to get the regular KDE menu bar back! I've done this.

And I configured my workspaces in a 3x3 grid, that way it doesn't take so much space (and also it's much easier to navigate them than 9 in a row!). This grid function is really one of the big things that annoys me in macOS (it used to have grids but they killed it when mission control came out), windows and Gnome. I need a grid, especially because I switch with hardware keys.