In Firefox I cannot connect to any website, proxy is disabled and outside network access is confirmed, no system or manual proxy was set on this Linux Mint/Ubuntu system. Normally this can be caused by proxy or DNS problems and the weird thing is that traceroute and ping to other IPs worked fine but even connecting to sites by IP was not working.
The connection was reset
The connection to the server was reset while the page was loading.
The site could be temporarily unavailable or too busy. Try again in a few moments.
If you are unable to load any pages, check your computer's network connection.
If your computer or network is protected by a firewall or proxy, make sure that Firefox is permitted to access the Web.
This even causes apt-get to fail to install packages
This was a virtual machine cloned in VirtualBox and for some silly reason it clones the MAC address, so there was the original machine running with the same MAC. The solution is to regenerate a new MAC address from the VBOX config.