Suddenyl I can't log in to wp-admin anymore?!?!

I was working on my site yesterday without a problem. Tonight I can't login. Every time I try to login, my login info just disappears, or it takes me to the default WP login screen where it shakes it's head at my attempts. I know my password is correct, but, out of frustration, I clicked on the lost password link. I tried entering both my email address and username, but both times when I hit Enter, my info just disappears. Sometimes it will take me to my front end profile page, which shows no other info, not even my sidebar, and the gray tool bar at the top no longer shows up.

This is all in Firefox. I'm using the latest release of WP, 3.3, plus the Nelo 2.1.7 theme.

Then I tried in IE. I got right in!

I tried to troubleshoot. I made a new admin and then was able to log in with that new user in FF. When I went back to IE and clicked on something, it took me to my logout screen. And I couldn't get back in.

I went back to FF and got as far as my blank front-end Profile page, and then copied in a backend URL directly and was able to get in.

When I go to my users page, I have myself, plus the new admin account, plus one subscriber (a friend of mine). When I click Edit on either the subscriber or the new admin account, it takes me to the regular user edit screen. When I click Edit on myself, it takes me to the blank front-end profile page. I was only able to enter into my regular user edit screen by editing the URL to enter in my user id.

Does anyone know why a perfectly functioning site would behave so strangely all of a sudden?

I have these plugins installed:
Author Avatars List
Contact Form 7
Dashboard: Last News
Executable PHP widget
Google Analytics Dashboard
Google XML Sitemaps
Jetpack by WordPress.com
Select Really Simple CAPTCHA
Select Theme My Login
Top 10
WPMU DEV Update Notifications

Please help!