Browser attempts to connect when already connected
From MozillaZine Knowledge Base
If Firefox or Mozilla Suite can't reach a site on Windows 2000 and XP, they will by default ask Windows to establish a connection. If you have multiple connections defined in Windows, this can lead to unwanted "Network Connections" dialogs (such as one for a VPN connection) when loading sites that are unreachable (site is down, ad blocker prevents loading). To resolve this issue, open about:config and set the preference network.autodial-helper.enabled to false.