Comment 2 for bug 1222912

I'm having a very similar issue here.

If I tell Firefox to use a proxy file (i.e. set network.proxy.autoconfig_url = file:///etc/proxy.pac and network.proxy.type = 2), and that pacfile exists, everything works fine, even if returning two proxies.

If I then rename or remove that proxy.pac, firefox goes along on its way merrily, but the IcedTea plugin fails. Running firefox from the cli yields the following output:

java version "1.7.0_79"
OpenJDK Runtime Environment (IcedTea 2.5.6) (7u79-2.5.6-0ubuntu1.12.04.1)
OpenJDK Client VM (build 24.79-b02, mixed mode, sharing)
Unable to use Firefox's proxy settings. Using "DIRECT" as proxy type.

(plugin-container:4609): GLib-WARNING **: Invalid UTF-8 passed to g_io_channel_write_chars().
/bui/dbuildd/icedtea-web-1.2.3/build7/../plugin/icedteanp/IcedTeaNPPlugin.cc:1859: thread 0xb1f2b560: Error: Failed to write bytes to output channel: Invalid byte sequence in conversion input