The Geneva Convntions do not prohibit the use of incendiary weapons. There are some minor restrictions, such as dropping incendiary bombs over urban areas, but that's it.

The reason that such weapons have generally dropped out of favour is that technology has progressed, and we have found more effective ways of doing the job. The napalm bomb has been replaced by the thermobaric bomb, and flamethrowers, being bulky, dangerous, short-ranged pieces of equipment, have been replaced by anything from precision bomb drops through man-portable missiles. Some missiles and unguided systems like Panzerfaust 3 have very good thermobaric and explosive rounds as well, and they are much better than flamethrowers for the same job.