You can use a self-signed cert but it will trigger SSL warnings in Firefox (and other browsers). If you're just testing your proxy, you can usually click through those warnings, but for anything more serious or production-related, it’s better to use a trusted certificate to avoid those hassles. If you want a free option that won’t give you warnings, check out Let’s Encrypt!

To get Let’s Encrypt, install Certbot, run it to obtain your certificate for your domain, and then configure your proxy to use those certificates. This way, you’ll avoid browser warnings and have a more professional setup!

Alternatively, you can get the cheapest commercial cert and install it on your server. It's a bit more complicated as you'll need to generate a CSR and pass Domain Control Validation, but it's just one of the options. So the choice is yours.