Thunderstorms, are convective storms, driven by convective heat transfer from lower layers of the atmosphere to higher up. During winter in temperate lattitudes the surface radiation balance (solar shortwave absorbed, minus net infrared radiation upwards) is negative or weakly positive, so very little convective activity ocurrs. While in the summer, the surface radiation balance is strongly positive, so the excess heat is carried upwards by convection.
Thunderstorms do not require a cold front, (although it helps). Thunderstorms are nearly a daily occurrence in some parts of the tropics.
I don't know if the frequency of thunderstorms will increase or decrease due to global warming. Convection is driven by temperature constrasts with respect to height, not to absolute temperature. There should be fewer cold fronts, because the poles will warm more than the tropics, this would tend to reduce the numbers. But, there will also be more moisture in the air which would tend to increase the numbers. The high end storms seem to be becoming more frequent, and this is thought to be becuse at higher temperatures the relative amount of latent energy of water vapor is higher, which tends to allow the strongest storms to sustain higher inflows thus drawing moisture in even faster.