Rated 5 out of 5 stars
Today I installed this add-on on my PC at my workplace , too.
I realized that add-on "Tool Bar Buttons 1.0" was formerly installed,
this YesScript icon was missing.
At such a time as this , temporarily disable Tool Bar Buttons and restart the Firefox.
Then YesScript icon will appear in the customize window.
Once YesScript icon is put on the appropriate place,
you may enable Tool Bar Buttons.