Firefox 4 worked perfectly for me, but Firefox 5 says a connection could not be established.
Today I upgraded from Firefox 4.0.1 to Firefox 5.0.1. That's when all of a sudden I was unable to go to any sites.
"Unable to connect
Firefox can't establish a connection to the server at [insert site here]
The site could be temporarily unavailable or too busy. Try again in a few moments.
If you are unable to load any pages, check your computer's network connection.
If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the Web."
Today I upgraded from Firefox 4.0.1 to Firefox 5.0.1. That's when all of a sudden I was unable to go to any sites.
"Unable to connect
Firefox can't establish a connection to the server at [insert site here]
The site could be temporarily unavailable or too busy. Try again in a few moments.
If you are unable to load any pages, check your computer's network connection.
If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the Web."
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It is best for you to read this article Firefox can't load websites but other browsers can
Maybe your firewall is blocking it. Some firewalls will block an application that is updated.