Thought so.
You are actually still "remembered" by the site. It's just that when you first bring it up on Firefox, it doesn't LOOK like you're remembered. And even filling out the log-in info may still take you back to the log-in page. Very confusing, I know.
Ignore all that. Come to the site as you usually do...then click on any of the links... Forum, Home, UserCP...whatever you usually use to start browsing the site. And you'll pop right in fully remembered and logged in.
(This issue has been seen on iPhone's as well...same fix...ignore the log-in screen and just click on a link to get started.)