I am not able to connect to internet using Firefox. I have Un-installed and reinstalled Firefox again, but still get the "Cannot find server" message. For now I'm using Internet Explorer but I would like to go back to using Firefox. Thanks.

I lost connectivity to internet thru Firefox a while ago. Contacted my ISP and we tried many things before discovering it was a Firefox issue, not a modem or router or browser issue. Even after uninstalling/einstalling Firefox fresh, I still cannot connect to internet and get the "Unable to connect- Firefox can't establish a connection to the server at www.yahoo.com" message.

A possible cause is security software (firewall) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.
Remove all rules for Firefox from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.
See:
*https://support.mozilla.com/kb/Server+not+found
*https://support.mozilla.com/kb/Firewalls

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