A while back I entered a password into a website - firefox then asked me if I wanted it to remember the passord for me. I selected the option to never offer to remember passords for this saite again... Now I have changed my mind - I'd love firefox to remember my password for this site, but I have no idea how to undo my previous selection.
Now that you've configured Firefox to remember passwords, try logging in to the site again.
Go to : Tools > Options > Security
Under passwords section:
Make sure "Remember passwords for websites is checked", then click the exceptions button and delete your site from that list.
Note: For Ubuntu and some other types of linux, Tools > Options is Edit > Preferences