Comment 16 for bug 557640

What is happening here is that as Firefox is starting, something initializes nsIOService before libdbusservice.so has registered nsNetworkManagerListener. Because there is no compreg.dat at this state, nothing provides NS_NETWORK_LINK_SERVICE_CONTRACTID at the point that this code section is executed in nsIOService::Init

    // go into managed mode if we can
    mNetworkLinkService = do_GetService(NS_NETWORK_LINK_SERVICE_CONTRACTID);
    if (!mNetworkLinkService)
        mManageOfflineStatus = PR_FALSE;

    if (mManageOfflineStatus)
        TrackNetworkLinkStatusForOffline();

So, from this point, Firefox does no tracking of the network connectivity and nsIOService::GetOffline always returns FALSE (which is what ubufox is using to get the network status).

When you run Firefox for a second time, calling do_GetService(NS_NETWORK_LINK_SERVICE_CONTRACTID); from nsIOService::Init triggers loading of libdbusservice.so and automatic registration of nsNetworkManagerListener, because it exists in compreg.dat from the previous run

XPCOM component registration has changed quite a lot in Firefox 4.0, so I suspect this might not be an issue there.

This isn't going to be fixed in Firefox 3.6.