Your proxy settings will depend on your provider but if you search for network.proxy
you’ll see all the setting you need in there. You’ll connect either through http (ip+port) or socks. Socks adds an additional layer of authentication which is generally preferred but likely won’t be offered by free options.
Yes, your proxy service will absolutely see what sites you are visiting. None of these suggestions will protect you from a malicious proxy service. Https is still honored assuming the end point is using a valid cert and your proxy has not tampered with it in any way. If the cert gets altered you’ll get a warning from your browser, you then have to trust yourself to know when to back out.
What are you using to auto connect to VPN when you disconnect from your home wifi?