I posted my question on the firefox.com site, in the support section. Still no replies but there are more people with the problem.
But, there's good news and there's bad news:
The bad news is that I can no longer help troubleshoot the problem for the other folks who have the problem.
The good news is that it went away. I mean just disappeared. It no longer happens.
At first I thought I had fixed it. I was looking at the list of plugins, as they are often the source of problems with Firefox, and noticed there were two Microsoft Office 10 plugins installed and enabled. I had previously uninstalled, and with the help of the good folks on this site, completely removed the bleeping Office 10 teaser HP had installed but apparently these two plugins got left behind. I disabled them and checked the login issue, and lo and behold, it was fixed - the login was successful without the 'requested format' message. Eureka!
I then went to the wife's Toshiba laptop, which had the same problem, to disable the plugins there also (one difference - the regular uninstall of Office 10 on her computer had been successful). Guess what - there were no Office 10 plugins. I then checked the login function to ensure the problem still existed, and it was gone on her computer as well.
So, I went back to my HP, enabled the plugins and found the login still worked. Today, after full shutdowns and cold boots on both machines, the problem is still gone, so I'm saying it's fixed. I have no idea why. Maybe the trick is to just go look at the plugins. Don't do anything, just open the page and look at them.