Firefox 55.0.2 how to change taskbar icon?

I use Firefox Portable 55.0.2 on Windows 10. I just spent about 2 hours looking for a way to change the taskbar icon. I want to set a custom taskbar icon for every instance of Firefox Portable that I run.

I tend to run about 5 Firefoxes at the same time, and it becomes hard to know which one is which. So I would like to set a custom taskbar icon for each, make the taskbar icon a simple number, and that will be that.

So my taskbar will look like this:

1 | 2 | 3 | 4 | 5

Instead of like this:

FF | FF | FF | FF | FF

Here is what I'm talking about - https://i.imgur.com/ZhHUdNV.png

I would rather do this by editing the Firefox Portable files/folders somehow. But even a solution to where I use Windows 10 or some 3rd party program to set the taskbar icon for each portable window would work.

How can I do this?

Thanks