While it is definitely true that some add-ons get a wider coverage and more downloads than others it is still a fact that each Firefox user had his own share of add-ons that he considers essential. This can be mostly attributed to different motives in using Firefox. A web developers is using a different set of add-ons than someone who loves to surf on video portals or social networks.
There is still some common ground, add-ons that are used by most of the Firefox community, or at least the part that knows that add-ons are available to extent the browser. Most users would call them essential but there will be some who will question the addition of one or the other add-on in the list.