To use a proxy for specific sites only, the easiest option is to create a pac file and configure Firefox to use it as automatic proxy configuration.

Example pac file:

function FindProxyForURL(url, host) {
    if (dnsDomainIs(host, "example.com")) {
        return "HTTP 192.168.0.1:8888";
    }
}

Store this somewhere and go to about:preferences → general → Network settings (very bottom) → settings, select “Automatic proxy configuration URL” and enter the path to the pac file in the file protocol notation, e.g.:

file:///home/user/.mozilla/firefox/ffox.default/custom_proxies.pac

After making changes to the file, go back to the Firefox proxy configuration URL settings and click reload.

Further info:
Mozilla Dev