First download the waterfox .tar file from https://www.waterfox.net/download/.
Then move it to a secured folder from the downloads folder. Then extract it there.
Search in the app menu if you can find a "Main menu" software.
If you find it, then click on it and click on "internet". Then click on "New item". Then set a name for the app (for example, Waterfox).
Then click on the icon which will take you to your file system. Then go to the folder in which you have kept the extracted waterfox file. Then from there you will go to,
Waterfox > browser > chrome > icon > defaults > default128.png
The icon will be selected.
Then click on browse beside the option command. It will take you again to your file system. Go to the folder in which you have kept the extracted waterfox file. From there you will go to Waterfox and scroll down and you will find a file named waterfox which is a shared library (application/x-sharedlib) type file. Then click on it. Then you will be again on the main page of "Main menu" . Now select "ok". And then go to your menu and search for waterfox. Then you can add it to your desktop by a left click of mouse on the app and then select pin to desktop.
If you do not find "Main menu" named app then you will have to install it.
This procedure was recently tested in Ubuntu MATE 20.04. So hopefully it will work.