The only way of MAYBE having a consistent experience on linux is to only use applications written for the DE of your choice isn't it? Use firefox on KDE and the consistency is already gone.
Is that any different to Firefox on macOS? In a way Firefox's current UI design aligns with the win 10 modern stuff, but that's more luck for MS than Firefox making a big effort to fit in, since it's mostly the same everywhere
No it hasn't. Just buy a phone from let's say Samsung and install any Google app. Google app will have a different UI look and feel than rest of the system.