The Guide shows you Step-by-Step How to Create a New Application Launcher inside to the Main Menu on Linux DescentOS Mate Desktop.
The Practical Example Here Illustrated is About Adding the Firefox Nightly Entry inside to the Internet Section a Linux DescentOS Mate Desktop.
The Technique is Useful for Adding Any New Software on a Linux DescentOS Mate Desktop.
To the Post Bottom there is a Link to Guide for Installing the Firefox-Nightly Browser on Linux DescentOS.

Right Click on Main Menu >> Edit Menu
On Main Menu
Select the Applications Group for the New Laucher
Select New Item
On the Create Launcher Select the Icon
Here Below Link to Article with Tutorial 4 Locating Files/Icons Under Linux.
Linux How to Locate CommandHow to Easy Find/Search Files/Directories/Images on Linux.
Linux How to Locate IconsAfter Complete the Launcher Parameters with Executable Path
Confirm with OK!.
Save the New Created Application Launcher
Next you can Use the New Launcher
How to Install Firefox Nightly in Linux DescentOS: