Thats because flatpaks treat each apps directory as their own $HOME so instead of $HOME/.mozilla its $HOME/.var/app/{app_name}/.mozilla
Which is still fantastic dont get me wrong. But Mozilla hasn’t stopped hardcoding their Mozilla folder instead of the xdg dirs even throughf firefox issue tracker has had it on there for 20 years
Basically all KDE apps have the same dependency set. So install one and the next ones will only install the app most likely. On KDE itself you’d already have these.