If you allow your browser to save passwords on your Mac, you can easily find out what that password is using the
security command at the command line.
security find-internet-password -a <your user name> -s <the domain> -g | tr -d '\n'
For instance, if I wanted to see what my password is for my GitHub account (provided that I saved that information in my Keychain) I would enter this at the command line:
security find-internet-password -a jacaetevha -s github.com -g | tr -d '\n'