Unfortunately, my understanding was that it was designed this way for security considerations. It's not just FF, it's any browser (although I wouldn't be surprised if FF has a plugin which remembers the content you placed into fields so that you don't have to fill out the password each time).
I posted a complaint about this in the feedback group in the communities a long time ago. If you want, you can add your voice to it.
One (bad) workaround for this is going into the MyNI section and into the registered products page. This page remembers your login info (at least the email address) and after you log in there you're also logged into the communities section.
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