A conditionalist says "eternal" doesn't always mean duration, that it can refer to quality. He uses the eternal fire of Jude 7 to support his position since the fire isn't still burning now.
"Now I desire to remind you, though you know all things once for all, that the Lord, after saving a people out of the land of Egypt, subsequently destroyed those who did not believe. And angels who did not keep their own domain, but abandoned their proper abode, He has kept in eternal bonds under darkness for the judgment of the great day, just as Sodom and Gomorrah and the cities around them, since they in the same way as these indulged in gross immorality and went after strange flesh, are exhibited as an example in undergoing the punishment of eternal fire ... But you, beloved, building yourselves up on your most holy faith, praying in the Holy Spirit, keep yourselves in the love of God, waiting anxiously for the mercy of our Lord Jesus Christ to eternal life" (Jude 5-7,20-21).
Notice that "eternal" is used three times in Jude. If Jude is using "eternal" to refer to quality, then the fallen angels are not going to be bound forever, nor will the righteous live forever. Any change to the concept of hell also requires the same concept of heaven. "These will go away into eternal punishment, but the righteous into eternal life" (Matthew 25:46). The same Greek word is used in Matthew as it was used in Jude.
The sinners of Sodom, Gomorrah, Admah, and Zeboiim were placed under a sentence or punishment by God. Jude's point is that we need to realize that God is willing and able to punish sinners. Their punishment brought them to face the eternal fires of Hell -- in other words, the destruction by fire was not their end but just the beginning of what they earned by their sins. Yes, the fire here on earth burnt out, and their earthly bodies were destroyed, but that was not their final end. When Jesus told the story of the rich man and Lazarus, notice the state of the rich man: "In Hades he lifted up his eyes, being in torment, and saw Abraham far away and Lazarus in his bosom. And he cried out and said, 'Father Abraham, have mercy on me, and send Lazarus so that he may dip the tip of his finger in water and cool off my tongue, for I am in agony in this flame'" (Luke 16:23-24). Those who died in Sodom and the other cities went from death by fire to existence in the fire in Hades and eventually, on Judgment Day they will be sentenced to eternal punishment in Hell.