I kind of feel this way about tabs, but everyone loves tabs so much I’m often too embarrassed to say anything.
Plasma (KDE) and Windows can both stack lots of windows just fine, so, why not let the window manager handle this and keep the browser more focused on browser tasks?
When you put
about:support
into the Firefox URL bar, do you see evidence that your GPU is being used? I’m not sure which settings, exactly, would indicate that, but mine, under “GPU #1” has “Active: Yes.”Lots of GPU-related options are also disabled, but I still get a smooth Google Earth experience.