Those forks aren’t maintaining Firefox itself, just their own modifications. If a bug is found in Firefox, the LibreWolf team don’t have to fix it themselves, they can wait for Mozilla to do it, and incorporate the fix once it materialises. There are forks that diverge further, but they either get quickly abandoned after their creator realises how much of a headache maintenance will be, or they’re left with gaping security holes.
Those forks aren’t maintaining Firefox itself, just their own modifications. If a bug is found in Firefox, the LibreWolf team don’t have to fix it themselves, they can wait for Mozilla to do it, and incorporate the fix once it materialises. There are forks that diverge further, but they either get quickly abandoned after their creator realises how much of a headache maintenance will be, or they’re left with gaping security holes.