GTK Styles and Fonts doesn't seem to work with Firefox.

I would like very much to change the fonts that Firefox uses from Sans-Serif to a Serif font. All of the fixes for this issue that I've found on the web advocate using GTK-QT-engine and GTK Styles and Fonts to fix this, but it's not working for me. After verifying that GTK-QT-engine (trinity-gtk-qt-engine) is installed, I shut down all instances of Firefox, then used the Trinity Control Center to set "Use my TDE style in GTK2 applications", "Use my TDE style in GTK3 applications", and "Use my TDE fonts in GTK2 applications"; but after restarting Firefox, the same old Sans-Serif fonts are displayed in all Firefox menus, dialogs, etc. I also tried "Use another (GTK2) font, specifying "Droid Serif (size 9)", but that didn't work either. There is no GTK3 Fonts section in GTK Styles and Fonts, so I can't explicitly tell TDE to change the GTK3 fonts. Is there another way to tell Firefox (and other GTK apps) to use a different font without installing all of Gnome? Leslie