Thank you for the support, I appreciate it!

Actually, that have been possible at least since 2011 :)

Right, makes sense.
Combining options weren't designed to work with the "Combine when taskbar is full" configuration.

With "Combine when taskbar is full", Windows decides when to combine/decombine groups, according to the available free space, and so the tweaker options for combining have no effect.

I can think of only one explanation: perhaps you changed Firefox to specifically not combine.

1. If you right click on Firefox's AppId (here), are there any configured settings?
2. If you open multiple windows of a different app (say, Notepad), do you still see multiple items? 

That's strange, with "Combine grouped buttons", there should be only one Firefox item, not three. Can you open Taskbar Inspector when there are several open Firefox windows open, and post here the screenshot?

Why did you uncheck "Don't group pinned items"? Sorry for the misleading picture, I meant to refer only to the highlighted options. Please recheck "Don't group pinned items" and see if that's what you're looking for.

Did you try the options that I suggested in my previous comment? It should achieve what you're asking for.

You selected "Don't group", and so each Firefox button is independent on your taskbar. Refer to the help file of the tweaker for more details.

I didn't fully understand what you're trying to achieve. Can you post your tweaker settings?

If you want the two Firefox buttons to look like a single item, enable grouping and combining:

Oh, I actually implemented support for this scenario, the installer is launched as elevated:

https://tweaker.userecho.com/topics/286-automatic-updates-failed-when-uac-is-enabled

Currently, options are either in HKCU (non-portable) or in an ini file (portable). If you install 7+ Taskbar Tweaker in Program Files (non-portable), each user still has his own settings.