Find a way to ship custom default bookmarks without changing language-packs on macOS

With multi-lingual packages we found that we are changing language packs to customize default bookmarks on macOS (#21879 (closed)).

However, the preference that allowed us to so does not seem to work anymore, and Firefox is showing a lot of warnings.

We should see if we can change the way to customize bookmarks.

Original description

I'm not sure if this is just a quirk of the testbuild, but the current state of about:addons in the multi-locale build is very... warning-y. To add insult to injury, the link behind the buttons all 404 too:

https://support.mozilla.org/1/firefox/102.3.0/Darwin/all/unsigned-addons