I am sorry, a formatting error caused my post to be cut short. Here is the rest of the question:

...I believe I have seen the same question here previously but it has been deleted after the asker failed to hide the "Other Bookmarks" folder and went for a workaround. I would not mind the folder but sadly, I cannot find the original question (answered by jscher2000 or cor-el, if I remember correctly) anymore. Other custom rules, such as

#PersonalToolbar {
	background-color: #000000 !important;
	color: #808080 !important;
	font-size: 6pt !important;
}

visibility: visible !important;

Normally, full screen works like this:

• No toolbars
• When toolbars are rolled down, no Bookmarks Toolbar

Do you want the Bookmarks Toolbar to show when the toolbars are rolled down, or all the time?

Do you want the Bookmarks Toolbar to show when the toolbars are rolled down, or all the time?

All the time.

Hmm, I can either show the bar when toolbars are rolled down, or when they are not rolled down, but not both.

In other words, you can use either:

/*
   Allow Bookmarks Toolbar in F11 full screen when toolbars are rolled down
*/
#navigator-toolbox[inFullscreen="true"] #PersonalToolbar {
  visibility: unset !important;
}

or

/*
   Allow Bookmarks Toolbar in F11 full screen when toolbars are rolled up
*/
#navigator-toolbox[inFullscreen="true"][style*="margin-top"] #PersonalToolbar {
  visibility: unset !important;
}

but not both.

I don't know how to get both.

I accidentally edited out a bit of information on why it's tricky:

Firefox now detects that the Bookmarks Toolbar was shown when the toolbars were rolled down and increases the negative top margin to hide it when going back to full screen, preventing it from sticking down into view.

visibility: visible !important;

I will leave the question marked as unanswered to prevent the thread from locking in case somebody figures out how to force visibility in either case. Personally, my issue was fixed, though.

By the way, for people running a real public-facing kiosk, using this is probably a bad idea since people might add bookmarks by dragging links from the webpage to the Bookmarks Toolbar, and exploiting the feature especially if no URL filtering or other restrictions are present. You may get away with it by rebooting regularly (shut down for the night) and using a tmpfs "RAM disk" - a popular option among Raspberry Pi users where the system disk is write-disabled, and the computer always boots into the same state as after your setup.