Rated 4 out of 5 stars
I assumed I had disabled the function way back when, but thought that Firefox could do this without any add-on. Apparently not [??].
So this is a good addition. I would have chosen 5 stars if there were a second access route to setting the default suggestion engine via its 'Options' button in the Add-ons list. Or if there had been, on this web page, some explanation for the sentence, "It is possible to set a default search engine which will be used for suggestions."
The 'Manage Search Engine List' dialog is just not that obvious a place to look, IMHO, and the Image Gallery on this web page is more of a hiding place than a display of the option's location.