Firefox can't establish a connection to the server

I have just change a new notebook. After a day of transferring data onto it, I started to use it. Everything seems to work fine until I was browsing on Firefox. A few sites I get the error "Firefox can't establish a connection to the server at websitename.com". I search through the web & forum but did not managed to get a solution. My proxy setting seems to be alright. I can go to those sites using IE and they is not an IE only sites. At the end of the day I gave up. I have a look at it again the next day and found the culprit. I have set the proxy for http but not for other protocol. So a simple checked on "Use this proxy server for all protocols" solved the problem. Come to think of it, the sites that I had problem accessing are https site which use SSL. How could I miss that? Sad to hear that a person (from a forum) gave up using Firefox because couldn't get this tiny issue fixed. Hope this help those who face the same problem out there.