The filament of a filament lamp is metal (tungsten), but the resistance does not remain constant. The graph of current against resistance is curved. As the current and PD rise the energy rate of energy change (power) in the  filament increases and the temperature rises. The resistance therefore increases and so the gradient of the graph (which is the inverse of resistance) falls. This causes the free electrons to become more excited and they no longer flow as easily, therefore raising the resistance.

A thermistor is made from a semi-conductor. These materials conduct electricity better when they are hot (but never as well as a metal). It is only the free electrons that will conduct a current. In semiconductors there are a few, but not many. (Metals have lots). When cold there are hardly any free electrons to conduct a current. When hot some of the electrons are sufficiently excited to move into higher energy levels, able to move from atom to atom. These free electrons can then conduct a current. The hotter the material is the greater the number of free electrons, allowing a current to flow more easily, reducing the resistance.