I somehow got it working adequately at home; it even honors "remember me" (which I could never get to work before). And I can use either the "forums" url or the "forums/usercp" url.
But now it's bad at work.
If I use the "forums" url, I go to the forums page ... put my creds in the boxes at top-right ... press "login" and I go to the same page ... no welcome at the top ... "if this is your first visit ...". But I am logged in; if I go to any group, I see the "Welcome, you last visited ...". But if I then click "usercp" I get what's described below (as if I had used the "usercp" url from the beginning.
If I use the usercp url, the first thing I see is the "vBulletin Message" page ... "You are not logged in ..." (this is not the "bad credentials page"). That page has two opportinuties to login (small, top-right and a big one in the middle) ... using either one takes me right back to the same page. I can "jump" to the forums and discover that I'm logged in ("Welcome vefatica") but if I click "usercp" I'm right back at the "You are not logged in ..." page.
I've had FF4 for a few days so I don't blame it for the problems which started yesterday.