A simple golang script to parse the ffldb database files created by btcd.

btcd_ffldb_parser.go

package main

import (
	"encoding/binary"
	"flag"
	"fmt"

	_ "github.com/btcsuite/btcd/database/ffldb"

	"github.com/btcsuite/btcd/chaincfg/chainhash"
	"github.com/btcsuite/btcd/database"
	"github.com/btcsuite/btcd/wire"
	"github.com/btcsuite/btcutil"
)

func main() {
	dbPath := flag.String(
		"dbpath",
		"/home/ubuntu/.btcd/mainnet/blocks_ffldb",
		"the btcd blocks ffldb file path",
	)
	startHeight := flag.Int(
		"start", 0, "specify the blocks starting height",
	)
	endHeight := flag.Int(
		"end", 0, "specify the blocks ending height. inclusive",
	)

	flag.Parse()

	blocks, err := FetchBlocks(
		*dbPath, int32(*startHeight), int32(*endHeight),
	)
	if err != nil {
		fmt.Println(err)
		return
	}
	for _, b := range blocks {
		fmt.Printf("found %v\n", b.Height())
	}
	fmt.Printf("found %v blocks\n", len(blocks))
}

var (
	// heightIndexBucketName is the name of the db bucket used to house to
	// the block height -> block hash index.
	heightIndexBucketName = []byte("heightidx")
)

// FetchBlocks opens the database and parses the blocks.
func FetchBlocks(dbPath string, start, end int32) ([]*btcutil.Block, error) {
	// load btcd db file
	db, err := database.Open("ffldb", dbPath, wire.MainNet)
	if err != nil {
		return nil, err
	}
	defer db.Close()

	return dbFetchBlocks(db, start, end)
}

func dbFetchBlocks(db database.DB, start, end int32) ([]*btcutil.Block, error) {
	var blocks []*btcutil.Block
	var err error

	err = db.View(func(tx database.Tx) error {
		blocks, err = bulkFetchBlocks(tx, start, end)
		return err
	})
	if err != nil {
		return nil, err
	}

	return blocks, nil
}

// bulkFetchBlocks uses an existing database bucket to retrieve a
// collection of block hashes for the provided height range.
// Note that the start and end values are inclusive. For instance, start=0,
// end=10 will fetch for block 0 till block 10, a total of 11 blocks.
func bulkFetchBlocks(tx database.Tx,
	start, end int32) ([]*btcutil.Block, error) {

	meta := tx.Metadata()
	heightIndexBucket := meta.Bucket(heightIndexBucketName)

	// make a slice to avoid copy
	blocks := make([]*btcutil.Block, end-start+1)

	// fetchHash is a closure to fetch block hash by a given height.
	fetchHash := func(height int32) (*chainhash.Hash, error) {
		var serializedHeight [4]byte
		binary.LittleEndian.PutUint32(
			serializedHeight[:], uint32(height),
		)

		hashBytes := heightIndexBucket.Get(serializedHeight[:])
		if hashBytes == nil {
			return nil, fmt.Errorf(
				"no block at height %d exists", height,
			)
		}

		var hash chainhash.Hash
		copy(hash[:], hashBytes)
		return &hash, nil
	}

	fetchBlock := func(hash *chainhash.Hash) (*btcutil.Block, error) {
		// Use the View function of the database to perform a managed
		// read-only transaction and fetch the block stored.
		blockBytes, err := tx.FetchBlock(hash)
		if err != nil {
			return nil, err
		}

		block, err := btcutil.NewBlockFromBytes(blockBytes)
		if err != nil {
			return nil, err
		}
		return block, nil
	}

	for i := range blocks {
		// Calculate the height and encode
		height := start + int32(i)

		hash, err := fetchHash(height)
		if err != nil {
			return nil, err
		}

		block, err := fetchBlock(hash)
		if err != nil {
			return nil, err
		}

		// assign the height
		block.SetHeight(height)

		blocks[i] = block
	}

	return blocks, nil
}

Get the following error when running this on the testnet:
block data for block 683e86bd5c6d110d91b94b97137ba6bfe02dbbdb8e3dff722a669b5d69d77af6 is for the wrong network - got 303307798, want 3669344250
how can this script be changed to support test net?

Get the following error when running this on the testnet:
block data for block 683e86bd5c6d110d91b94b97137ba6bfe02dbbdb8e3dff722a669b5d69d77af6 is for the wrong network - got 303307798, want 3669344250
how can this script be changed to support test net?

I think you only need to point the dbpath to your testnet files, not mainnet/blocks_ffldb but testnet/blocks_ffldb.