There are just so many ways to disable the toolbars in Firefox, I wondered if you wouldn't be better off with a different browser?
Solution 1: Use Chrome and set the Home page to chrome://bookmarks. Even if he hides the bookmarks bar, he can still click on the Home button to open the bookmarks manager.
Note that you can also use Ctrl+Shift+B to toggle the bookmarks bar; in Firefox this displays the bookmarks manager.
Solution 2: Use Opera 28 or later. The bookmarks bar can only be enabled or disabled through Settings, so it's impossible to accidentally hide it.
Opera also has a proper 'tile menu' implementation, dubbed "SpeedDial", which may be all he needs in the way of bookmarks. Tip: to create a SpeedDial folder, create it in the bookmarks first, then right-click to send it to SpeedDial. I really like the way these folders work once they're set up.
On top of all that, there's a link to the bookmarks manager at the bottom of each new tab.
You can also do the Home button trick I suggested with Chrome, only the URL is opera://bookmarks, but I think the other features make that a little unnecessary.
Note that Opera is a completely different product to days of yore and is now based on Chromium, just like Chrome. I haven't used it for a long time, but it's starting to look quite interesting.