/usr/share/ubuntu-defaults-builder/template/webbrowser/bookmarks-toolbar.txt is in ubuntu-defaults-builder 0.30.
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 | # The following bookmarks will be appended to the default bookmark folders in the toolbar
# For now only Firefox is supported.
#
# Format: arbitrarily many lines with
# URL Name
#
# Example:
# http://www.zeit.de Die Zeit Nachrichten
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