New Florence was a Terran Hegemony world founded sometime between 2271 (HB:HMarik p 16) and 2366 (HB:HLiao p 17) annexed by the Draconis Combine in the First Succession War (HB:HLiao p 31) and then taken by the Capellan Confederation in the Second Succession War (HB:HLiao p 39). It vanished from maps between 2864 and 3025 (HB:HLiao p 40).
New Florence was located between Ronel and Tybalt. I'm not aware of anything having been published that indicates why the world vanished off the maps, but that's not uncommon. The fact that it vanished off maps in the Third Succession War rather than in the First or Second would seem to imply that it was a victim of something other that weapons of mass destruction, given that I can't think of many cases in canon where worlds were attacked with such weapons during that war - the era has been described as seeing lower levels of warfare than the planet-eating destruction of the first two succession wars. Although there's always the possibility that it was hit with WMDs during the First or Second Succession War, and simply became ecologically untenable - we've seen that happen with worlds during the Jihad, like Radstadt. When I wrote the New Florence entry on Sarna, I didn't attribute its vanishing from maps to any particular cause as I'm not aware of one having been cited in canon, so it seemed inappropriate to speculate. If there's something out there that contradicts it, a reference would be handy because I could incorporate that information into Sarna.