Did you try to continue without making changes?

Facebook may not have recognized your cookies correctly, but it might work if you ignore that warning.

Clear the cache and the cookies from sites that cause problems.

• "Clear the Cache": Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"
• "Remove the Cookies" from sites causing problems: Tools > Options > Privacy > Cookies: "Show Cookies"

• http://kb.mozillazine.org/Cookies
• http://kb.mozillazine.org/Websites_report_cookies_are_disabled

I had the same problem with FF3 and now FF4. Finally, today I tried "Tools, Options, Remember History" rather than "Tools, Options, use Custom Settings for History, Allow Cookies", and FB loads flawlessly. For some reason FB doesn't seem to recognize the custom settings.

Hey, finally got it! I tried the things that were mentioned above and they still didn't fix the problem...however, they were part of the solution.

Here's what I had to do:

Clear all cache and cookies as mentioned in previous wonderful answers. Close FF browser and restart. I have an igoogle homepage. After clearing cache and cookies the page wants me to sign back in. Well, this time, I did not sign in, I just went to Facebook.com, it has my login info in the top right corner, I clicked login, and voila Facebook is back. I can then go to my igoogle page and sign in, and still remain signed into Facebook. And now Facebook opens even when I close out of FF again and then restart. I don't have to go through the process again. However, I am going to keep these instructions for myself to refer to should it happen again.

Thanks to locoarchy and cor-el for taking the time to read and answer to my problem.

I love my FF!

Jen