Receive "Server not found" message inconsistently on various sites. Sometimes "retry" command is successful; frequently not successful. The "Server not found" message even occurs with Mozilla website links on this help location. Have cleared cookies & cash, restart in safe mode (disable add-ons). No success in eliminating the problem.
A possible cause is security software (firewall,anti-virus) 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 and the plugin-container 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.
I have this problem too, opening links starts having server not found page.
Has nothing to do with router or software.
If I close firefox and try again it works, but will start giving server not found after a few links