Firefox 43.0.4 does not send a User-Agent request header to youtube.com or ytimg.com if the "media.youtube-ua.override" pref = false. This problem does not affect FF 44+. How can a user tell whether they have the hotfix installed? In my two FF 43.0.4 test profiles (OS X and XP), I still see the "media.youtube-ua.override" = true, suggesting I haven't received the hotfix.

If it matters, I believe my FF 43 instances never had 43.0.3. I don't use the Release channel often, so I think I manually updated from 43.0 or 43.0.2.

There's a bug in the UA override in nsHttpHandler.cpp. We return nsHttpHandler::mYouTubeUserAgent in nsHttpHandler::UserAgent() for all "YouTube" URLs without checking the media.youtube-ua.override pref first. But we *do* check the pref when we initialize nsHttpHandler::mYouTubeUserAgent, and if the pref is false, we initialize mYouTubeUserAgent to the empty string. So if the pref is false, we'll return an empty UA string for "YouTube" URLs. We should have checked the cached pref in nsHttpHandler::UserAgent(). To mitigate this, we can either: 1. Ship another dot release to backout the ua hack (or to fix it, backing it out seems most sensible), or 2. ship a hotfix to toggle the media.youtube-ua.override pref to true, and set "media.youtube-ua.override.to" to "43". This means we'll still take the UA override rewrite path, but we'll rewrite "43" to "43" in the UA string for the HTTP request, i.e. not actually change the version in the UA string.

YouTube reports that the missing UA header causes the same EU Cookie problem like the UA override does and additionally breaks the YouTube gaming web app and 360 degree and 3D videos.

2. is not hard to do and this is a great idea. I'll update my hotfix in bug 1237103

I fixed that in another hotfix: https://hg.mozilla.org/releases/firefox-hotfixes/rev/5f3e2a43c8fa

Has this hotfix been pushed out? How much latency is there in hotfixes taking effect? YouTube are still reporting blank user agents.

The hotfix does not seem to be live. The latest published hotfix is version 20160106.01. Here is the server response from the hotfix update request: https://versioncheck-bg.addons.mozilla.org/update/VersionCheck.php?reqVersion=2&id=firefox-hotfix@mozilla.org&version=&maxAppVersion=%ITEM_MAXAPPVERSION%&status=userEnabled,incompatible&appID={ec8030f7-c20a-464f-9b0e-13a3a9e97384}&appVersion=43.0.4&appOS=WINNT&appABI=x86-msvc&locale=en-US&currentAppVersion=43.0.4&updateType=%UPDATE_TYPE%&compatMode=%COMPATIBILITY_MODE% <em:version>20160106.01</em:version> <em:updateLink>https://addons.cdn.mozilla.net/user-media/addons/354399/mozilla_firefox_hotfix-20160106.01-fx.xpi

This hotfix is now live. Even though the hotfix was built on 2016-01-12, its version number is still "20160106.01". If you have the hotfix installed, the "extensions.hotfix.lastVersion" pref should have a value >= "20160106.01". With this hotfix, I now see FF 43 sending FF 43 UA to youtube.com.

btw, my test profile did not receive the hotfix at first because my "app.update.auto" and "app.update.enabled" prefs had (somehow) been flipped to false. In a new profile, they were true, the default value.