B9sTool 3DS

Instructions

WARNING: Never, under any circumstance, use this homebrew in conjunction with a youtube/video guide of any kind!

This app only writes to FIRM0, not FIRM1, so it should be safe given your FIRM1 is not corrupt or a9lh'd.
Never use this on anything but 3DS FIRM 11.15.0-YZ
Never use this if sighax/b9s is already installed (on any firmware)! The result will be a sloppy uninstallation at best and a brick at worst
Be cautious when using this with arm9loaderhax installed, as the special handling for it is still experimental

Just supply the current decrypted FIRMs for both new/old 3ds and put them in the firm_new and firm_old directories respectively. Then place the newest boot9strap.firm payload in the payload directory. You will also need to supply a clean secret_sector.bin in the a9lh_files/secret_sector directory if you want the arm9loaderhax handling to work on n3ds systems without a the user supplying that file. Then compile with make FIRM_INFO="11.10 only" or whatever is actually the firmware range of the current native firm. The user does not have to supply any extra files like in previous versions. Needs the latest libfat version.

This has been removed. This feature was made in the interest of safety, but the realization that the update trick can reliably restore corrupted firms made it redundant.

It was also a pain in the ass.

This is dumped to your boot9strap folder when b9s is installed. It isn't used now but may be useful in the future. Don't delete it.

The hex number in the filename is the first 4 bytes of the file's SHA1.

When you install boot9strap multiple times, the standard behavior is that it will cycle between b9s and stock firm.

Don't count on this though, it's just a property of xoring operations and really isn't intended.
It is recommended you only install boot9strap once.

6.0.1 is for firmware 11.15 ONLY (always check the firmware in the app menu to be sure as this can change as new firmwares are released!)
5.0.2 - 6.0.0 is for firmware 11.13
4.0.1 - 5.0.1 is for firmwares 11.8 to 11.10 or 11.11(eur) ONLY
4.0.0 is for firmwares 11.4 to 11.7 ONLY
Be careful using b9sTool between firmware updates - only use b9sTool on the firmware it's intended for.