This GPO isn't as straightforward as with Chrome/Edge, as there appears to be a legacy Bookmark object, too. (Ignore the Bookmarks object, with the list of 50 bookmarks; use the Managed Bookmarks GPO referenced below.)
Objective:
Turn on the bookmarks toolbar, by default:
Mozilla's "Getting Started" and "Most Visited" folder
json
[
{
"toplevel_name": "Company Resources"
},
{
"url": "https://example.com",
"name": "Company Portal"
},
{
"name": "IT Helpdesk",
"children": [
{
"url": "https://announce.example.com",
"name": "IT Announcements"
},
{
"url": "https://tickets.example.com",
"name": "Open a Ticket"
}
]
}
]