Comment 2 for bug 390975

Have got the same problem. But it is incorrect to say that QT don't use subpixel rendering - in fact looks like that after installing KDE systemsettings and kdebase-workspace-bin fonts that are used by the firefox are controlled by KDE system settings, including KDE's ability to do subpixel rendering if properly configured for it. Affected parts of firefox include menu bars, popup menu, bookmarks toolbar and tabs selector, meanwhile page rendered and input boxes still use fonts from gnome-settings.