I've been trying to find an answer to this for years. Having openwrt and ddwrt images but no way to get them on the thing is annoying so here...
Buffalo encrypts their firmware. Their old one had been cracked, but nothing within the last several years. That leaves two options (at least on 1.70): serial or tftp. Keep in mind that getting back to stock will be nearly impossible unless you backed it up.
For tftp, your server must be on 192.168.11.168 and the file must be named firmware_WHR-1166DHP.ram (yes, I have the non-HP). It only loads the file into RAM, so the regular images can't find root and you'll have to flash a regular image later when you get past that. An image with an initramfs and the right command line worked for me for initial boot. I threw the one I made that finally worked for me up on google drive.
To put it in tftp boot mode you can hold down the aoss button while plugging it in and a bit after that (until you see it grab the image). Once it's up, you need a new IP. Backup the stock FW and sysupgrade (with a regular build) for a permanent install.