LXR shows that there are three prefs: pref("network.hosts.socks_server", ""); pref("network.hosts.socks_serverport", 1080); pref("network.hosts.socks_conf", ""); Based on my recollection w/ someone, these prefs are from circa Navigator 2 or 3, when SOCKS V4 support was added to Communicator. socks_conf support was dropped in Comm 4, and according to bug 134304, the socks_server pref might not even be the same in migrated profiles, it might be "socks.server". A search of all comments in bmo shows no bugs that discuss the implementation or usage of these prefs. LXR is similar, although it looks like photon is reading them (but possibly using the prefs in their defaults). ja-JP, trunk, and minotaur have these prefs, it looks like Phoenix and Chimera removed them already.

(here's the lxr output because it takes a long time:) network.hosts.socks /l10n/langpacks/nscp/ja-JP/defaults/pref/all.js, line 480 -- pref("network.hosts.socks_server", ""); /l10n/langpacks/nscp/ja-JP/defaults/pref/all.js, line 481 -- pref("network.hosts.socks_serverport", 1080); /l10n/langpacks/nscp/ja-JP/defaults/pref/all.js, line 482 -- pref("network.hosts.socks_conf", ""); /modules/libpref/src/init/all.js, line 612 -- pref("network.hosts.socks_server", ""); /modules/libpref/src/init/all.js, line 613 -- pref("network.hosts.socks_serverport", 1080); /modules/libpref/src/init/all.js, line 614 -- pref("network.hosts.socks_conf", ""); /embedding/browser/photon/src/PtMozilla.cpp, line 646 -- pref->SetCharPref( "network.hosts.socks_server", value ); /embedding/browser/photon/src/PtMozilla.cpp, line 648 -- pref->SetCharPref( "network.hosts.socks_serverport", value ); /embedding/browser/photon/src/PtMozilla.cpp, line 860 -- pref->CopyCharPref( "network.hosts.socks_server", &s ); /embedding/browser/photon/src/PtMozilla.cpp, line 865 -- pref->CopyCharPref( "network.hosts.socks_serverport", &s ); /mail/app/profile/all.js, line 596 -- pref("network.hosts.socks_server", ""); /mail/app/profile/all.js, line 597 -- pref("network.hosts.socks_serverport", 1080); /mail/app/profile/all.js, line 598 -- pref("network.hosts.socks_conf", ""); Patches for all but photon coming up in bug 205077. I also checked Comm for Mac, and confirmed that the old value in migrated profiles used the "_". The reference to "...hosts.server" was probably a typo on stephend's part.

UPDATE: pref is now covered in migration utility: http://lxr.mozilla.org/mozilla/source/browser/components/migration/src/nsDogbertProfileMigrator.cpp#267 and most files are now gone. Still need to to patch minimo. Photon appears to support these values, so I've created bug 237576. Thinking about this more, I don't know if, historically, the pref in communicator used these lines and we added SOCKS code w/ the wrong prefs.

Fixed via depends. I actually found a system of mine that still has Communicator 4, and confirmed that the prefs value was used in the old profile format.

stephend documents this in bug 134304, but migration is not version specific (see bug 246989).