I prefer using Firefox Nightly over the OS installed Firefox. I've downloaded it to a folder on my persistent home drive partition and have used it this way for several years. With every new OS install, I add it to the menu and assign it an icon from the app's icon folder.
I recently discovered the Latte Dock and really like it. So I removed the default FF icon from the dock and added the Nightly icon. Every time I reboot and launch Nightly, i get a second (like with any non-pinned app) icon on the right side of the dock the first time I launch it. After I close it, when I relaunch Nightly, it acts normally and shows a dot or line under the pinned icon. Any ideas on why this first launch action occurs or how to fix it?

I tried unpinning the pinned icon then using the unpinned icon to pin it back, but same actions occur.