PayPal Fix - Silver Bullet
I had the same issue with PayPal after making an Online purchase. I tried everything including opening up new windows and rebooting but it seemed to be locked on the error page no matter what. I could not log in or out of the Paypal error page using Firefox 3.6.8. I could however use IE to access the Paypal web site with no problem.
I called Paypal to get help and fortunately reached someone who knew how to fix it. The first thing is to make sure that cookies are enabled. My version of Firefox enables cookies by default so unless you have an exception for PayPal to not enable cookies it will always be enabled. Early versions of Firefox may be different so you should double check the settings for yourself. Also to be 100% sure that PayPal cookies specifically are enabled I was advised to manually add one to be certain it is not part of the problem. To do this go to Tools, Privacy and then from the pull down menu select Use Custom Settings for History. From there make sure that Accept Cookies from Sites and Accept Third-party Cookies is checked. Then click on the Exceptions button then under the window Address of Web Site type Paypal.com and then click on allow. After I did this I still had the same problem with loading Paypal but it may help depending on what is causing your issue.
The final thing to do which is the only thing that worked for me is to go to Tools, Clear Recent History, and then from the pull down menu select Everything and than click the Clear Now button. This was the Silver Bullet! Good Luck and hope it solves your issue