How to reset the enable password on an ASA 5505:

The following procedure worked for me to reset the enable password.

Connect to serial port – typically 9600,8,N,1.  On my MacBook Pro, I use a Keyspan USB-Serial adapter, so my command line is:

screen /dev/tty.USA19Hfd13P1.1 9600,8

You can eventually use <ctrl-A><ctrl-\> to kill the screen session.

Power on the device.
When it prompts to interrupt boot sequence, do so (press ESC).

It should prompt

rommon #0>

Type in:
rommon #0> confreg

Should show something like:

Current Configuration Register: 0×00000001
Configuration Summary:
boot default image from Flash

Do you wish to change this configuration? y/n [n]:

Press n (don’t change)

We can have the ASA boot a default config with no password by setting register flags 0×41, so do this:

rommon #2> confreg 0×41
rommon #2> reboot

You now can login as the password has been removed (use <return> as the password).  Be sure to set the enable password with:

config t
enable password new-password-here
config-register 0x1
wr

Ensure you either use the config-register command or interrupt the boot sequence again and reset the boot flags back to 0x1, otherwise the boot loader will continue to boot the default configuration – ignoring your configuration.