Do this answer your question?

https://support.mozilla.org/en-US/kb/pinned-tabs-keep-favorite-websites-open

Hi The Boss, do you mean on the "Jump List" displayed when you right-click the Firefox icon, or do you mean completely separately from the Firefox icon?

A good way to pin a URL to the Jump List is a two step process:

(A) Drag and drop the "lock" icon from the left end of the address bar to the Windows desktop to create a shortcut (if there's no lock, there may be an "i-in-a-circle" icon)

(B) Drag and drop the new shortcut to the Taskbar and Windows will offer to pin it to Firefox, assuming Firefox is your default browser

While it is possible to drag-and-drop the URL directly to the Firefox icon on the Taskbar, you don't get a nice title that way.