sorry for the delays, pressures of life... *shrugs*
Firefox was just updated to current version, 50.0.2, within the last half hour (It's been a while since I have used this computer).
Extracted from my current profile,
prefs.js, WITHOUT Firefox running at the time:
user_pref("extensions.dwhelper.advanced-settings", true);
user_pref("extensions.dwhelper.chunks.concurrent-downloads", 8);
user_pref("extensions.dwhelper.chunks.prefetch-count", 8);
user_pref("extensions.dwhelper.convertHelper.lastChecked", 1477434810);
user_pref("extensions.dwhelper.converter-aggreg-tune-h264", true);
user_pref("extensions.dwhelper.converter-path", "C:\\Program Files\\ConvertHelper3\\avconv.exe");
user_pref("extensions.dwhelper.converter-threads", "2");
user_pref("extensions.dwhelper.dlconv-last-output", "e6587753-4ca5-4d2e-b7ba-beaf1e7f191c");
user_pref("extensions.dwhelper.download-count", 8);
user_pref("extensions.dwhelper.download.controlled.max", 2);
user_pref("extensions.dwhelper.hls.download-as-m2ts", true);
user_pref("extensions.dwhelper.last-version", "6.1.1");
user_pref("extensions.dwhelper.need-prefs-migration", false);
user_pref("extensions.dwhelper.scrap.state", "stopped");
user_pref("extensions.dwhelper.storagedirectory", "C:\\Users\\scott_000\\dwhelper");
user_pref("extensions.dwhelper.toolbar-button", "maincomp");
user_pref("extensions.dwhelper.tpsr.state", "stopped");
I disabled VDH in extensions, reloaded Firefox. No VDH tab appeared.
I re-enabled VDH in extensions, reloaded Firefox, and got a tab accessing this webpage:
http://www.downloadhelper.net/update.php?from=5.6.0&to=6.1.1*facepalms*