Over the years, the amount of bookmarks that I like to keep in my toolbar has grown and they no longer fit n a single row. I searched a bit and found this very useful Firefox add-on, called Roomy Bookmarks Toolbar. It does exactly what it’s description says and does it well.
Once installed, I tweaked its settings (Tools > Add-ons > Preferences), to show only one row, which can be scrolled with the mouse wheel:
There was a glitch, that made the toolbar display a cut off part from the row beneath it. This was easily fixed by the following option:
Works like a charm!