On Firefox:

1. Go to about:config

2. Right-click anywhere, New -> Boolean:

signon.overrideAutocomplete

Value: true

More info: https://bugzilla.mozilla.org/show_bug.cgi?id=425145

EDIT: Based on the milestone set on that issue, this setting requires Firefox 29. Another workaround is this bookmarklet: https://www.squarefree.com/bookmarklets/forms.html#remember_... referenced from comment https://bugzilla.mozilla.org/show_bug.cgi?id=425145#c16