The only problem I have is that I end up keeping the toolbar hidden because my toolbar space is at a premium: that defeats the easy access to those search engines. As good an idea as this is, it would greatly benefit from a toolbar button for one-click access (two-clicks if you count clicking the search buttons themselves).
Feature request taken from user review:
https://addons.mozilla.org/en-US/firefox/addon/searchbuttonsbar/reviews/840766