It has only stated today after installing KDE's system settings and configuring it to make KDE apps look native . Firefox has started using a thinner font.
so how do i make it use the default fonts ?
For a fix, see the fourth item at http://www.webgapps.org/tutorials/firefox/troubleshooting/rendering-issues-and-solutions
If you want a simple way of changing Firefox font, then use Theme Font & Size Changer extension.