From OLPC website

You can change your preferences by creating a user.js file in /home/olpc/.sugar/default/gecko – the browsers profile folder.
The file should look like this:
pref(”network.proxy.http”, “your_proxy_name“);
pref(”network.proxy.http_port”, address);
pref(”network.proxy.type”,1);

To view your browser’s proxy settings: enter “about:config” into the browser’s url field.