Vaseline works well, as does vapor-rub. You can mix in various waxes, mineral spirits, kerosene, petrolatum to get the mix, temperature-viscosity curve, you want.

Point being that a gel is generally easiest to work with. A mix that is soft at -20F is a messy liquid at 80F and a mix that is handy at 80F is a solid block at -20F. Vaseline, by itself, soaked into cotton balls, assuming the Vaseline doesn't leak out and christen the contents of your pack, turns into a bottle of oil with cotton at the bottom in Florida summer heat.

A stopgap measure is to use more cotton and less Vaseline. But this means each ball burns a shorter time. Blending in wax, I add a splash of naphtha to get it all to mix and allow the mix to stand so the naphtha evaporates, allows you to have a stiffer mix at higher temperatures and still have a long-burning fire-starter. A lot depends on your comfort and ability at bathtub chemistry. Experiment with different stuff and find a formula that works for you.

An interesting variation that seems to burn a bit hotter is called pine tar. It is more like pine oil. It was a vital marine resource into the 20s and was used straight for keeping hemp ropes from rotting, stirred into oakum to caulk their boats, and combined with wax on both on boats and ashore to make canvas water resistant.