Investigate potential DNS leak on Windows when using SSPI
Firefox supports the "negotiate" auth method which, it seems, may lead to a DNS request bypassing the proxy settings in: https://mxr.mozilla.org/mozilla-esr24/source/extensions/auth/nsAuthSSPI.cpp#122. This is somewhat mitigated as it does not affect the majority of Windows users due to
being empty and
being set to "false" by default. Might be worth investigating further and fixing if this preliminary analysis is sound.