It depends on whether the bookmarks were synced or backed up. If you were signed into a Firefox account on the old iPhone, install Firefox on the new phone and sign in with the same account. Your bookmarks may sync back. Also check any other device where you use Firefox, like a computer or iPad. If the bookmarks are still there, don’t make changes yet — let that device sync them back first. If the deletion already synced everywhere, then Firefox may not be able to recover them unless you have an old phone backup or another device that still has them.