Answer by Ho1 for Firefox cannot load page in 16.04

You are probably experiencing a DNS problem, mostly caused by slow local DNS servers. You can find out your current DNS setting by invoking:

nmcli dev show | grep DNS

Try setting your DNS to something like 4.2.2.4.

For a better diagnosis, do the steps listed here:

Server Not Found - Troubleshoot connection problems

https://support.mozilla.org/en-US/kb/server-not-found-connection-problem

You should test other browsers, and upon the result you can find out exactly the root of problem. But according to what you have stated, the highest possiblity is the DNS problem. This can be remedied with DNS prefetching, although it has some drawbacks.