Firefox - Is there a way to pin a Firefox URL as a separate item on the Windows 10 taskbar?

While the entire process isn't quite as simple as it is with Chrome or Edge, it is still relatively easy:

1. Create a new Desktop shortcut that points to Firefox. The easiest method is to open the Firefox installation directory, right-click on firefox.exe, and select Create shortcut from the context menu. The default 64-bit Firefox installation directory is C:\Program Files\Mozilla Firefox, while the default 32-bit Firefox installation directory is C:\Program Files (x86)\Mozilla Firefox.
2. Select Yes when Windows informs you that the shortcut cannot be created in the Firefox directory. The shortcut is created on the desktop, instead.
   
3. Right-click on the new Firefox shortcut and select Properties from the menu.
4. Append the URL of the site that you want opened by the shortcut within the Target field, e.g., "C:\Program Files\Mozilla Firefox\firefox.exe" -url https://superuser.com/
   
5. Click on OK to save the changes.
6. Right-click on the desktop shortcut and select Rename to change it to something more appropriate. In our example, we decided on "Super User"
7. Right-click on the desktop shortcut again and select Pin to taskbar. Optionally, you may delete the desktop shortcut once you are done.
   
   

Sources:
Add your favorite websites to the Windows 10 Taskbar
How do I pin a Firefox shortcut to the taskbar in Windows 10?

Read More