/usr/lib/firefox/browser/defaults/preferences/vendor-firefox.js is in firefox 59.0.2+build1-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | // Disable default browser checking.
pref("browser.shell.checkDefaultBrowser", false);
// Don't disable extensions dropped in to a system
// location, or those owned by the application
pref("extensions.autoDisableScopes", 3);
// Don't display the one-off addon selection dialog when
// upgrading from a version of Firefox older than 8.0
pref("extensions.shownSelectionUI", true);
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