how to configure firefox to use proxy?

I am using proxy server and I configure the IE8 to use proxy and it is working fine while the firefox not working also I configure to use proxy , it is giving this error
Explanation: There is a problem with the page you are trying to reach and it cannot be displayed.

Try the following:

    Refresh page: Search for the page again by clicking the Refresh button. The timeout may have occurred due to Internet congestion.
    Check spelling: Check that you typed the Web page address correctly. The address may have been mistyped.
    Access from a link: If there is a link to the page you are looking for, try accessing the page from that link.

If you are still not able to view the requested page, try contacting your administrator or Helpdesk.

      Technical Information (for support personnel)

    Error Code: 502 Proxy Error. The request was rejected by the HTTP filter. Contact your ISA Server administrator. (12217)
    IP Address: 10.1.1.11
    Date: 12/20/2011 5:28:51 AM
    Server: dhow-vpn.DHOWSOFT.COM
    Source: web filter
IhabAsked:
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IhabAuthor Commented:
Thanks Eyal for your Kind support I think it is Isa server Issue
0
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