Can't open a certain website – which proxy for Firefox should I use?

"Do you mean lists of proxys you can use? A proxy doesn't care what browser you use so you don't need to search for ""mozilla proxies"".

Unless you pay for your proxy account, it is very likely that using the proxies you find out in the wild is not exactly legal and slow.

As Molly said, you really might want to have a look at Tor."

It could be that you have a spyware/virus infection. These often block updates to Microsoft.com, to stop you downloading updates.