The fires that were occasionally lit in the literal Gehenna, or Tophet, were necessarily only temporary. They died out for want of fuel. Worm dieth not, . . . An expression borrowed from the last verse of Isaiah , and probably in current use among the Jews of our Savior's time, as applied to the state of future retribution. Our Lord is alluding to the pervading idea of verses 45-48 .
Fire is used in the Scripture to denote suffering, persecution, trial, distress of any kind.
Salt is used to denote permanence, preservation from corruption. Every one, good and bad, must suffer. Have salt in yourselves.