Add "GTK Styles and Fonts" to to fix Thunderbird/Firefox appearance

Hi all,

I'm running RHEL 6.5 x86-64, fully updated as of today and KDE.

I noticed that the fonts used by Thunderbird and Firefox look really poor even though I have forced 96 DPI in "System Settings => Appearance => Fonts".

Everything else looks good so I believe this to be a GTK issue.

On a SUSE system I have running KDE, under "System Settings => Appearance" there is also "GTK Styles and Fonts" but this is not present under RHEL.

How can add this?

If not possible, how can I control/fix the fonts used by Thunderbird/Firefox?

Many thanks for your time!
Ken