"Use System Proxy Settings" is almost always the correct proxy setting for Firefox. (In fact, this is the only proxy configuration that IE and Chrome support.) But, when we migrate from IE, we switch from "Use System Proxy Settings" to "No proxy" if the system isn't configured to use a proxy.
Instead, when the migration tool is migrating browser options, it should always reset the proxy items to "Use System Proxy Settings." This way, any kind of auto-configuration that is done (e.g. using connection management tools like Lenovo's ThinkVantage Access Connections) will work for Firefox automatically.
I've made this change already on the latest patch on bug 710895. See bug 710895 comment 11 for details.
The difference is that I removed all the code that imports network preferences. Since "Use system proxy settings" is the default it doesn't try to set it. Do you think we should go even further and actively reset it to "use the system settings" if it had been changed?
we don't import anymore these settings.