Thunderbird and Firefox are GTK+ apps.  GTK+ is a windowing toolkit that GNU/Linux application developers tend to use when creating software on the GNOME Desktop Environment.

If you decide to switch to an alternative desktop environment, such as KDE, the default settings for GNOME/GTK applications may be ignored.  This is because KDE uses the Qt windowing toolkit instead.

To fix this, you need to do it tell KDE to pick up the GTK settings and apply them to your GTK apps.

In Fedora/CentOS, this is simple:

# yum install qtcurve-gtk2.x86_64

Then in KDE, point to Kicker (the application menu) > system settings > Application Appearance > GTK+ Appearance

Change the Widget Style (dropdown) from Redmond to qtCurve.

More information for Ubuntu users is here: