i cant add tools to the overflow menu (im talking about the BUILT IN ones like zoom controls)

(srry if wrong topic) basicly when i use the customize firefox tab it tells me about the overflow menu but when i try to add BUILT-IN tools like zoom control it just, not, and also the overflow icon apears gray'ed out,

if someone can help me with this then pls reply to this, ill come back to this post like a week later