The bookmark toolbar has 3 options.
Always show; Never show; Only show on new tabs.
The latter option does not work.
Users have reported that “Only show on new tabs” refers to tabs displaying the built-in new tab page, and that it may not work on other pages such as an extension page, or on a completely blank page.
I think the blank page scenario is on file in Bugzilla, but I’m not sure whether a decision has been reached about how to safely detect the desired scenario (about:blank is used for frames and other situations where it would not be helpful to generate the toolbar).
If you want to get as close as possible to blank, you can assign the “Firefox Home” page and uncheck all of the various items that can populate the page.
See: https://support.mozilla.org/kb/hide-or-display-content-new-tab
So, load a page from a server on the other side of the world, run by people I don’t know and have no specific reason to trust, every time I open a new window. No thanks.
How about only generating it once on start up; and just hiding it until needed.
And then unhiding it when a user clicks 
or types ctrl-T???
{further comment elided}
?? The “Firefox Home” page is the built-in new tab page.
Then one wonders why “Blank page” isn’t the “New tab” page? (Ie. the “FireFox Home” without all the … let’s be polite and call them … extras.
That would fix the OP problem instantly without forcing extra coding to detect when About:blank shoudl and should not make the Bookmark toolbar appear.