Firefox/Thunderbird interface too big
 

This problem has always existed for me. Both thunderbird and firefox's interface font is way too big. How do I decrease the size? I have tried to use the preferences menu but I do believe that it only affects the message text size. Any help?

maybe changing system font will help. in kde, usually at preferences (look & feel)> font

My gtk theme is set to size 8 and the theme in firefox is about a 16.