There is nothing to copy:

"Go to the extension settings and copy the address"

There is no address to copy:

My empty page' is set to 'home page'.

Since I could not do what Gleb Khegay suggested, I opened a fresh session of Firefox and opened Speed Dial. Then I opened Firefox options and set Home Page to current page (Speed Dial).

This resulted in this being entered into Home Page:

moz-extension://1a35cd14-4293-4b84-8d9b-1f9357e75420/newtab.html

I closed Firefox.

Now when I open Firefox I get an empty page and have to open a new tab which then displays Speed Dial.