Assuming you are using the new GTK+ Appearance theme QtCurve settings...
System Settings -> Appearance -> Widget Style -> QtCurve -> Configure... Sliders and Scrollbars... Under "Groove appearance", uncheck "Thin".
Now the Firefox scrollbars should look less weird (ie normal). Be sure to change your widget style back to Oxygen before leaving the dialog. Or discard the changes... it will still keep the QtCurve settings change I believe since you "Ok"ed them in the previous menu... that should really be fixed, IMO. Kinda not too user friendly.
System Settings -> Appearance -> Widget Style -> QtCurve -> Configure... Sliders and Scrollbars... Under "Groove appearance", uncheck "Thin".
Now the Firefox scrollbars should look less weird (ie normal). Be sure to change your widget style back to Oxygen before leaving the dialog. Or discard the changes... it will still keep the QtCurve settings change I believe since you "Ok"ed them in the previous menu... that should really be fixed, IMO. Kinda not too user friendly.