After I enter the web site, a warning comes up that says a browser at the web site is already open and says .../multisession.asp
First, you should update to Firefox 14 (Update Firefox to the latest version).
Then, if you are still having trouble, I'd suggest the following to Reset Firefox:
Further information can be found in the Refresh Firefox - reset add-ons and settings article.
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Check the connection settings.
If you do not need to use a proxy to connect to internet then try to select "No Proxy" if the "Use the system proxy settings" setting or one of the others doesn't work properly.
See "Firefox connection settings":