Bookmark toolbar visibility on new tabs is not honored when new tab page is not about:blank

STR:

1. Right click on the bookmark toolbar.
2. Make sure its visibility is set to "Only show on New Tab".
3. Open a new tab.

Result:

If you check about:config,browser.toolbars.bookmarks.visibility is flipped but the toolbar is not visible.

Notes:

• Also affecting Mullvad Browser: https://github.com/mullvad/mullvad-browser/issues/14.
• @thorin says Windows is working just fine. I can test Linux later today, if needed.
• IMO it makes sense to have browser.toolbars.bookmarks.visibility set to never whenever letterboxing is enabled.