Are you sure about the counting? Years ago it looked like they count the same add-on in multiple categories, and in reality there were a only a few thousands, not 10s of thousands, and yes, I already excluded things like color-schemes, personas, and even themes.

edit: That was before the discontinueing support of XUL-based extensions. I can't imagine there are more webex-based now. Except when adding the endless persona/colorscheme stuff which adds no real functionality, only changing some looks.

Or, they count the symlinks, instead of the original file.