Tails

The issue is also present in Tails 3.9, so not a regression in Tails 3.9.1 with Tor Browser 8.0.2.

So the spoofing has been uplifted from Torbutton to Firefox. The pref is privacy.spoof_english, with (from Tor Browser sources, browser/app/profile/firefox.js):

// 0 - will prompt
// 1 - don't spoof
// 2 - spoof

Those settings do what they are supposed to in Tor Browser outside of Tails, but when set to 1 or 2 they are always "User set" (user_pref(...)), but in Tails we change the default to 2. We've seen buggy prefs before that behave differently for the same value depending if they are set via a default or user set.

FYI most automated tests involving Tor Browser are tagged fragile so they won't run on Jenkins unless your branch's name has the +force-all-tests suffix.

So, simply adding

user_pref("privacy.spoof_english", 2);

to

config/chroot_local-includes/etc/tor-browser/profile/prefs.js

did not work, the HTTP_ACCEPT headers still include the non-EN locale.

I took a look at the code today. The only place I could find where the spoofing is applied is here, in LanguagePrompt.jsm. There it seems like the spoofing is only applied if the preference is changed, so it doesn't matter what the value is when firefox is started.

This is consistent with the behavior I see: If I manually change the setting to 1 and then back to 2, the spoofing is enabled.

I also noticed that when the spoofing is applied, it actually only sets two other preferences. I guess if we would set these in config/chroot_local-includes/etc/tor-browser/profile/prefs.js, we could enable the spoofing again.

Built and tested, seems to work