Firefox normally uses GTK and KDE uses QT libraries. So KDE does not always control Firefox. In order to do that you often need to edit your GTK theme. Or you could build Firefox against QT. You see in Linux you are not locked into one supplier. But then the onus of making the correct choice is on you.

What the Linux community doesn't care about is ignorant users. So make some kind of an effort and educate yourself! Freedom isn't free. Or you can just keep sucking on whatever gets shoved down your throat.