Did you check your security software (firewall)?

A possible cause is security software (firewall) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process.

See:

• Server Not Found - Troubleshoot connection problems
• Configure firewalls so that Firefox can access the Internet

That would be logical and I checked that. But if the problem were the firewall, it wouldn't have allowed the browser to connect and authenticate to that website in the first place. And it wouldn't be allowing Firefox to access other sites. The disconnect/reset happens after authentication on certain banking sites. This wasn't a problem until Firefox 4.0.

I had the exact problem. It seems when connecting from firefox i was getting directed to onlineeast2 and then the connection would reset. When connecting from IE i was getting directed to onlineeast3 and the connection was fine. So I edited my hosts file to force onlineeast2 to resolve to the ip address of onlineeast3. This is working for me. here is what i put in my hosts file:

171.159.66.159 onlineeast2.bankofamerica.com

171.159.66.159 onlineeast3.bankofamerica.com

The hosts file can be found here: C:\WINDOWS\system32\drivers\etc\hosts