How can I add a Firefox bookmark as toolbar button?

"This may not be exactly answering your question,
But, if you want to save the bookmark toolbar space (and not just hide and unhide it at times),
you can reduce the bookmark toolbar content and move it completely besides the 'Help' button.
This way you are done with just two 'bars' under the firefox title.
The Menu bar and the Address bar...
Does that work for you?

To get this done,

right-click in the space besides the Help button

select 'customize...'
drag the bookmarks toolbar from its position to the new one besides 'Help'
end customization with the 'Done' button
Now, if your bookmark has a good icon, you can even delete its name string and
make it look quite like a button there..."