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

Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"

"Remove Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"

Try the following solutions and whichever setting works, keep it Permanent:

1) go to Tools Menu -> Options -> Advanced -> Network -> click "Settings..." button infront of Connection -> select option1 -> Click OK -> OK

2) go to Tools Menu -> Options -> Advanced -> Network -> click "Settings..." button infront of Connection -> select option2 -> Click OK -> OK

3) go to Tools Menu -> Options -> Advanced -> Network -> click "Settings..." button infront of Connection -> select option3 -> Click OK -> OK

Check and tell which solution works.