I figured this out in the end. The problem is that the startup() entry point for bootstrapped extensions gets called by the addon manager before the chrome manifests for other extensions (including language packs) have been loaded. Because this extension does something with starts nsIHttpHandler when it loads, nsIHttpHandler initializes the Accept-Language value before the chrome for the en-GB language pack has been loaded (so it ends up with the fallback value for en-US, with that being the only provider registered at that point)
** Changed in: firefox (Ubuntu) Importance: Undecided => Low ** Changed in: firefox (Ubuntu) Status: Invalid => Triaged -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/867753 Title: Accept-Language header contains incorrect value To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/867753/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs