I don't use a proxy server to connect to the internet, but Firefox automatically configures itself to use the loopback address.
I have a direct connection to the internet and don't use a proxy server. Every few weeks, Firefox automatically resets itself to be configured to use a proxy server at the loopback address (127.0.0.1). Manually changing it back to "No proxy" works, but I have to do so each time I open the browser or even a new tab. The only fix I can find is to create a new profile which gives me a new, clean install of FF, but then I lose all my add-ons. This has happened repeatedly now and it's extremely frustrating. I have scanned for malware but have not found any problems.