firefox-nightly double icon + firefox-beta -> Wrong icon

[Icy-Thought]

Firstly, I would like to thank you for creating this overlay and reducing the time/effort we have to spend for us to install Firefox nightly on NixOS!

Secondly, I'd like to enlighten you about several issues that I have been facing while using this overlay.

Issues

Firefox Nightly:

1. The icon shown under Gnome application menu is labelled as "Firefox" and when launched from Gnome taskbar another firefox-nightly is created with the label "Nightly" in the non-pinned applications area.
   

2. Right-click menu for the "Firefox" icon does not have an alternative for launching a "Private Window".
   

I installed Firefox Beta to check whether the icon work and apparently the icon Firefox-beta uses is the same as Firefox-nightly whereas the icon seems to be working as expected.

Steps to replicate the process

1. Use the following import as a nixpkg-overlay:

import (builtins.fetchTarball {
 url = https://github.com/mozilla/nixpkgs-mozilla/archive/master.tar.gz;
})

2. Add latest.firefox-nightly-bin to either configuration.nix or home.nix.
   a.Tried both and both packages were the same even after using sudo nix-collect-garbage -d.